fix: support directory runfiles in Windows launcher

[alex-the-third]

Summary

Support Windows remote execution environments where Bazel provides a runfiles directory without a manifest.

The generated launcher currently forces RUNFILES_MANIFEST_ONLY=1, and its :rlocation routine requires a manifest. Simply removing that override is insufficient because the non-manifest path is returned unchanged instead of being resolved beneath RUNFILES_DIR.

This change:

• uses RUNFILES_MANIFEST_FILE when it identifies a valid manifest
• otherwise resolves runfiles relative to a valid RUNFILES_DIR
• falls back to the usual adjacent .runfiles/MANIFEST, .runfiles_manifest, and .runfiles locations
• updates the exported runfiles variables so downstream Bash runfiles lookups use the selected layout
• removes the unconditional RUNFILES_MANIFEST_ONLY=1 override
• adds targeted Windows tests for explicit and adjacent manifest/directory layouts and their precedence

This matches Bazel’s native Windows launcher behavior while keeping the change scoped to the manifest-to-directory fallback tracked by the linked issue.

Fixes #1273

Comments

This extracts the gist of #872.

This should be the followup of #1272 as the first commit here is from the other PR. EDIT: the PR was merged, so rebased.

[jbedard]

@codex 🙏

[chatgpt-codex-connector]

To use Codex here, create an environment for this repo.