Make FileRequired faster by using HasSuffix instead of filepath.Base

[vpasdf]

The path in FileRequired has / even on windows. This is because the walk function uses path.Join.
http://google3/third_party/scalibr/extractor/filesystem/internal/walkdir_iterate.go;l=73;rcl=680571378

Using HasSuffix is O(len(suffix)) rather than O(len(path)) for filepath.Base.

path usually does not have a / prefix. So 1:1 replacement of filepath.Base(p) == x to strings.HasSuffix(p, x) won't work. But something like strings.HasSuffix("/"+p, x) could work. It's unclear how much the string concatenation costs.

This needs more benchmarking and one refactoring of the code base.