Resolver: allow drive-less absolute paths on Windows

Summary:
Fix a mostly Meta-specific edge case in Windows resolution. When using absolute paths without a drive letter on Windows, eg `\my\project`, don't use `path.resolve` to normalise to system separators, as this has the unwanted side-effect of prepending the the CWD's drive letter, even when the input is known to be absolute (according to `path.isAbsolute`):

On a Windows machine:
```
C:\> node -p "path.isAbsolute('/my/project')"
true

C:\> node -p "path.isAbsolute('\\my\\project')"
true

C:\> node -p "path.resolve('/my/project')"
C:\my\project
```

This preserves drive-less original paths. It also makes absolute path handling slightly stricter on Windows by not fully normalising the input (e.g., not normalising `C:\foo/../foo/.//bar` to `C:\foo\bar`, but instead using a simple `replaceAll`), which brings it into line with posix path handling - I'm considering this a "fix" rather than breaking, as behaviour for poorly-formed inputs is undefined.

Changelog:
```
 - **[Fix]**: Treat absolute path specifiers on Windows with the same strictness as posix, and allow absolute paths without drive letters.
```

Reviewed By: huntie

Differential Revision: D63175565

fbshipit-source-id: 35ab129f4846579dfe6ae8860d6d49b09cadc4e8

packages/metro-resolver/src/PackageResolve.js

@@ -134,7 +134,9 @@ export function redirectModulePath(
       // BRITTLE ASSUMPTION: This is always treated as a package-relative path
       // and is converted back, even if the redirected path is a specifier
       // referring to another package.
-      redirectedPath = path.resolve(containingPackage.rootPath, redirectedPath);
+      redirectedPath = path.isAbsolute(redirectedPath)
+        ? path.normalize(redirectedPath)
+        : path.join(containingPackage.rootPath, redirectedPath);
     }
   } else {
     // Otherwise, `modulePath` may be an unprefixed relative path or a bare

packages/metro-resolver/src/resolve.js

@@ -220,8 +220,11 @@ function resolveModulePath(
   toModuleName: string,
   platform: string | null,
 ): Result<Resolution, FileAndDirCandidates> {
+  // System-separated absolute path
   const modulePath = path.isAbsolute(toModuleName)
-    ? resolveWindowsPath(toModuleName)
+    ? path.sep === '/'
+      ? toModuleName
+      : toModuleName.replaceAll('/', '\\')
     : path.join(path.dirname(context.originModulePath), toModuleName);
   const redirectedPath = context.redirectModulePath(modulePath);
   if (redirectedPath === false) {
@@ -584,16 +587,6 @@ function resolveSourceFileForExt(
   return null;
 }
 
-// HasteFS stores paths with backslashes on Windows, this ensures the path is in
-// the proper format. Will also add drive letter if not present so `/root` will
-// resolve to `C:\root`. Noop on other platforms.
-function resolveWindowsPath(modulePath: string) {
-  if (path.sep !== '\\') {
-    return modulePath;
-  }
-  return path.resolve(modulePath);
-}
-
 function isRelativeImport(filePath: string) {
   return /^[.][.]?(?:[/]|$)/.test(filePath);
 }