Only inject metadata for outputs that cannot be reconstructed by skyframe later

[coeuvre]

Currently, they are symlinks.

For other outputs, let skyframe read action file system to construct the metadata.

Before this change, we inject metadata of symlink outputs, tree outputs and file outputs inside RemoteActionFileSystem#flush() if these outputs are stored inside the in-memory fs. If the outputs are somehow generated in the local fs, skyframe will read the fs later to construct metadata for them.

However, for tree outputs, skyframe always create an empty directory before action execution in the in-memory fs. So inside the flush, we always inject metadata for it. It means local tree outputs are ignored. We could update the code to also read local file system when reading tree files, but then the problem is how to construct metadata from file status which is well done by skyframe.

So instead of injecting metadata by traversal the filesystem inside flush, we just let skyframe to do the job.

Fixes #16789.

[gregmagolan]

Confirmed that this change fixes the problem observed on rules_js with Bazel 6.0.0rc1 & rc2. aspect-build/rules_js#659 updates rules_js to last_green and CI is green with --remote_download_toplevel set.

Thanks for the quick fix @coeuvre !