Add regression test for relative imports

Invocations of `nixpkgs_package` with a Nix file that refers to other
files by relative path into a separate directory, e.g. `./foo/bar.nix`,
or `../baz.nix` fail with `No such file or directory`.

The reason is that `nixpkgs_package` will flatten the directory
hierarchy of files defined in `nix_file_deps`.

See the following error:

```
$ nix-shell --pure --run 'bazel test //tests:run-relative-imports'
ERROR: /home/aj/tweag.io/da/bazel-projects/rules_nixpkgs/tests/BUILD.bazel:7:5: no such package '@relative-imports//': Traceback (most recent call last):
        File "/home/aj/tweag.io/da/bazel-projects/rules_nixpkgs/nixpkgs/nixpkgs.bzl", line 149
                _execute_or_fail(repository_ctx, nix_build, failure_m...), ...)
        File "/home/aj/tweag.io/da/bazel-projects/rules_nixpkgs/nixpkgs/nixpkgs.bzl", line 318, in _execute_or_fail
                fail("\n{failure_message}\nCommand: {...))

Cannot build Nix attribute 'hello'.
Command: [/nix/store/zw52rvbzhh6x447yc0hn3w7s0wmcq3pw-nix-2.2.2/bin/nix-build, "-A", "hello", "--out-link", "bazel-support/nix-out-link", "-I", "nixpkgs=/home/aj/.cache/bazel/_bazel_aj/cf7431ac3fb5fc6f4c6a5569d7b55e4a/external/nixpkgs/nixpkgs"]
Return code: 1
Error output:
error: getting status of '/home/aj/.cache/bazel/_bazel_aj/cf7431ac3fb5fc6f4c6a5569d7b55e4a/external/relative-imports/relative_imports/nixpkgs.nix': No such file or directory

 and referenced by '//tests:run-relative-imports'
ERROR: Analysis of target '//tests:run-relative-imports' failed; build aborted: no such package '@relative-imports//': Traceback (most recent call last):
        File "/home/aj/tweag.io/da/bazel-projects/rules_nixpkgs/nixpkgs/nixpkgs.bzl", line 149
                _execute_or_fail(repository_ctx, nix_build, failure_m...), ...)
        File "/home/aj/tweag.io/da/bazel-projects/rules_nixpkgs/nixpkgs/nixpkgs.bzl", line 318, in _execute_or_fail
                fail("\n{failure_message}\nCommand: {...))

Cannot build Nix attribute 'hello'.
Command: [/nix/store/zw52rvbzhh6x447yc0hn3w7s0wmcq3pw-nix-2.2.2/bin/nix-build, "-A", "hello", "--out-link", "bazel-support/nix-out-link", "-I", "nixpkgs=/home/aj/.cache/bazel/_bazel_aj/cf7431ac3fb5fc6f4c6a5569d7b55e4a/external/nixpkgs/nixpkgs"]
Return code: 1
Error output:
error: getting status of '/home/aj/.cache/bazel/_bazel_aj/cf7431ac3fb5fc6f4c6a5569d7b55e4a/external/relative-imports/relative_imports/nixpkgs.nix': No such file or directory
```

WORKSPACE

@@ -96,4 +96,15 @@ filegroup(
     repository = "@nixpkgs",
 )
 
+nixpkgs_package(
+    name = "relative-imports",
+    attribute_path = "hello",
+    nix_file = "//tests:relative_imports.nix",
+    nix_file_deps = [
+        "//:nixpkgs.nix",
+        "//tests:relative_imports/nixpkgs.nix",
+    ],
+    repository = "@nixpkgs",
+)
+
 nixpkgs_cc_configure(repository = "@remote_nixpkgs")

tests/BUILD.bazel

@@ -19,6 +19,7 @@ package(default_testonly = 1)
         "nix-file-test",
         "nix-file-deps-test",
         "nixpkgs-git-repository-test",
+        "relative-imports",
     ]
 ] + [
     # These tests use the nix package generated by ./output.nix

tests/relative_imports.nix

@@ -0,0 +1,4 @@
+{ system ? builtins.currentSystem
+, pkgs ? import relative_imports/nixpkgs.nix { inherit system; config = {}; overlays = []; }
+}:
+{ inherit (pkgs) hello; }

tests/relative_imports/nixpkgs.nix

@@ -0,0 +1 @@
+import ../../nixpkgs.nix