We currently have Path defined as std::string. This is kinda sloppy in general, but especially bad if we ever want to work on Windows.

We should instead use std::filesystem::path, which is the newish C++ standard Path type, and which does all the right things on all platforms.

Unfortunately, there isn't yet a std::filesystem::path_view. cplusplus/papers#406 will eventually fix this, but it is not yet stable and won't be for a while. This gives us two choices:

1. Use https://github.com/ned14/llfio, which is pioneering what will hopefully be standardized as std::filesystem::path_view.
2. Hand-roll our own PathView, which can be a small convenience around std::basic_string_view<Path::value_type> that has the missing implicit coercions.

I think (2) is the better first step; we can easily got from (2) to (1) later if we want.

Also per what @edolstra says before CanonPath should not change and we should also use it more. It would be for "internal" paths / paths in our SourcePath VFS we want almost all file access to go though. That means std::filesystem::path is just used for the lower level glue code with the OS.