Get rid of /rustc/$hash hack

[pnkfelix]

Spawned off of a8e4236

Right now, /rustc/$hash is used as a magic prefix to track paths into libstd source so that it can be remapped to local developer paths when they install the rust-src component.

But, as @eddyb pointed out on PR #72767, once we are willing to actually allocate another entry in the FileName enum (or its moral equivalent, splitting the payload of one of its variants into two via a separate RealFileName enum), then we should be able to do away with the /rustc/$hash prefix hack entirely within the rustc source code.

[eddyb]

The problem is having a way to cause the "virtualized" case to be used. Perhaps remapping to something more like remapping to rustc:$hash (which isn't a valid path AFAIK) instead of /rustc/$hash.

There's also the possibility that distros may choose to do something with /rustc/$hash (since it will show up in debuginfo), although I'm not sure what.