Adds tests for the pack list generation (#5873)

**What's the problem this PR addresses?**

#5872 made me notice that we have few tests covering how the `*` / `**`
patterns are interpreted.

**How did you fix it?**

This diff fixes that by adding a couple of tests, along with comments on
the few places where we seem to diverge from npm. It doesn't try to
change the pack list generation, just to increase its coverage and make
it easier to add more tests in the future.

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.yarn/versions/5452f3ea.yml

@@ -0,0 +1,2 @@
+declined:
+  - "@yarnpkg/core"

packages/acceptance-tests/pkg-tests-specs/sources/commands/pack.test.js

@@ -1,6 +1,14 @@
-import {xfs, npath}    from '@yarnpkg/fslib';
-import {fs as fsUtils} from 'pkg-tests-core';
-import tar             from 'tar';
+import {xfs, npath}          from '@yarnpkg/fslib';
+import {fs as fsUtils, misc} from 'pkg-tests-core';
+import tar                   from 'tar';
+
+async function genPackList(run) {
+  const {stdout} = await run(`pack`, `--dry-run`, `--json`);
+
+  return misc.parseJsonStream(stdout).map(entry => {
+    return entry.location;
+  }).filter(location => !!location);
+}
 
 describe(`Commands`, () => {
   describe(`pack`, () => {
@@ -316,6 +324,151 @@ describe(`Commands`, () => {
       }),
     );
 
+    test(
+      `it should support gitignore patterns (*)`,
+      makeTemporaryEnv({}, async ({path, run, source}) => {
+        await fsUtils.writeFile(`${path}/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/.gitignore`, `x.js\n`);
+
+        await run(`install`);
+
+        await expect(genPackList(run)).resolves.toEqual([
+          `foo/y.js`,
+          `package.json`,
+          `y.js`,
+        ]);
+      }),
+    );
+
+    test(
+      `it should allow to explicitly add back a file excluded by a gitignore`,
+      makeTemporaryEnv({}, async ({path, run, source}) => {
+        await fsUtils.writeFile(`${path}/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/.gitignore`, `*.js\n!x.js\n`);
+
+        await run(`install`);
+
+        await expect(genPackList(run)).resolves.toEqual([
+          `foo/x.js`,
+          `package.json`,
+          `x.js`,
+        ]);
+      }),
+    );
+
+    test(
+      `it should allow to explicitly add back a single file excluded by a gitignore`,
+      makeTemporaryEnv({}, async ({path, run, source}) => {
+        await fsUtils.writeFile(`${path}/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/.gitignore`, `*.js\n!/x.js\n`);
+
+        await run(`install`);
+
+        await expect(genPackList(run)).resolves.toEqual([
+          `package.json`,
+          `x.js`,
+        ]);
+      }),
+    );
+
+    test(
+      `it should support gitignore patterns (**)`,
+      makeTemporaryEnv({}, async ({path, run, source}) => {
+        await fsUtils.writeFile(`${path}/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/bar/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/bar/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/.gitignore`, `**/x.js\n`);
+
+        await run(`install`);
+
+        await expect(genPackList(run)).resolves.toEqual([
+          `foo/bar/y.js`,
+          `foo/y.js`,
+          `package.json`,
+          `x.js`, // TODO: This shouldn't be here; https://github.com/yarnpkg/berry/issues/5872
+          `y.js`,
+        ]);
+      }),
+    );
+
+    test(
+      `it should keep digging inside excluded folders (unlike gitignore!)`,
+      makeTemporaryEnv({}, async ({path, run, source}) => {
+        await fsUtils.writeFile(`${path}/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/bar/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/bar/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/.gitignore`, `*\n!x.js\n`);
+
+        await run(`install`);
+
+        await expect(genPackList(run)).resolves.toEqual([
+          `foo/bar/x.js`,
+          `foo/x.js`,
+          `package.json`,
+          `x.js`,
+        ]);
+      }),
+    );
+
+    test(
+      `it should keep digging inside excluded folders (unlike gitignore!)`,
+      makeTemporaryEnv({}, async ({path, run, source}) => {
+        await fsUtils.writeFile(`${path}/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/bar/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/bar/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/.gitignore`, `*\n!x.js\n`);
+
+        await run(`install`);
+
+        await expect(genPackList(run)).resolves.toEqual([
+          `foo/bar/x.js`,
+          `foo/x.js`,
+          `package.json`,
+          `x.js`,
+        ]);
+      }),
+    );
+
+    // TODO: https://github.com/yarnpkg/berry/issues/5872
+    test.skip(
+      `it should be possible to re-exclude something which got not-excluded`,
+      makeTemporaryEnv({}, async ({path, run, source}) => {
+        await fsUtils.writeFile(`${path}/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/bar/x.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/foo/bar/y.js`, `module.exports = 42;\n`);
+        await fsUtils.writeFile(`${path}/.gitignore`, `*\n!x.js\nfoo/bar\n`);
+
+        await run(`install`);
+
+        await expect(genPackList(run)).resolves.toEqual([
+          `foo/x.js`,
+          `package.json`,
+          `x.js`,
+        ]);
+      }),
+    );
+
     test(
       `it should override fields in the packed manifest`,
       makeTemporaryEnv({