fix(esbuild-plugin-pnp): respect resolveDir

[jenseng]

On-load callbacks can provide a custom resolveDir for virtual modules; if set, it should be used for resolving paths in that file. importer is only guaranteed to be a file system path if the namespace is file.

For context, see: https://esbuild.github.io/plugins/#on-resolve-arguments

What's the problem this PR addresses?

If another esbuild plugin creates virtual modules with relative imports, esbuild-plugin-pnp will incorrectly resolve them, since it assumes importer (the virtual module) is a real file path, and there are no such guarantees. This bug was observed after integrating esbuild-plugin-pnp into remix core and attempting to convert virtual modules to use relative imports.

How did you fix it?

Updated esbuild-plugin-pnp to use resolveDir (if set):

• For modules in the file namespace, this is equivalent to path.dirname(importer), so the behavior there is unchanged
• For plugins that set a resolveDir, esbuild-plugin-pnp will now pick it up and use it <- this is the fix
• For plugins that don't set a resolveDir, esbuild-plugin-pnp will still fall back to importer and baseDir

Also added an end-to-end test to verify the behavior (fails on master, passes with the fix).

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.