proposal: os: make os.dirFS implement fs.ReadFileFS and fs.ReadDirFS

[bvisness]

The fs.FS returned by os.DirFS implements only fs.FS and fs.StatFS. It seems like it could easily implement fs.ReadFileFS and fs.ReadDirFS as well, but it currently does not.

My use case for this is trying to use a common interface for both embed.FS and os.DirFS, to load files on the fly in development but embed them in production. This is difficult, since embed.FS implements fs.ReadFileFS and fs.ReadDirFS, but os.DirFS implements only fs.StatFS.

I have implemented fs.ReadDirFS for myself and it seems to work fine. From reading other issues (#49580, #47803), I don't think this would have any issues with the restrictions around paths in fs.FS either.

[seankhliao]

Not sure if this needs a proposal, I think we've agreed to do ReadFile in #53761

[rsc]

Closing as duplicate of #53761.

[rsc]

This proposal is a duplicate of a previously discussed proposal, as noted above,
and there is no significant new information to justify reopening the discussion.
The issue has therefore been declined as a duplicate.
— rsc for the proposal review group