fix: allow mapfs to open dirs

[knqyf263]

Description

mapfs.Open() should work with directories.

A File provides access to a single file. The File interface is the minimum implementation required of the file. Directory files should also implement ReadDirFile. A file may implement io.ReaderAt or io.Seeker as optimizations.

https://pkg.go.dev/io/fs#File

This is a leftover task from #3654. This PR adds support for directories that implement ReadDirFile.

Checklist

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• I've added tests that prove my fix is effective or that my feature works.
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