filemapping: add support for io/fs (including e.g. go:embed filesystems)

[moqmar]

This allows to e.g. use //go:embed locales to build the locales into the binary.

[mvo5]

Thanks, this looks good. I wonder if you could add some unit test (also to see how this will be used in practice ). I wonder for example if "LocalFS" should be exported or part of some New* helper and seeing how you use it or envision to use it would be helpful here :)

[moqmar]

I think LocaleFS & LocalePath belong really closely together, I don't see why one would be exported and the other not, so no need to change the API IMO.

I've added the FS usage to the example with go:embed to the README.

[mvo5]

Thanks for the test and the README.md addition. This looks very nice. Fwiw, I think it's fine if we bump from go-1.13 to go-1.18 in go.mod if you want to do a test that tests the go:embed behavior

[moqmar]

All done 👍

Edit: now finally all tests should pass, I obviously have a linux machine :D

[mvo5]

Thank you!