Also move `SourcePath` into `libutil`.
These changes allow `error.hh` and `error.cc` to access source path and
position information, which we can use to produce better error messages
(for example, we could consider omitting filenames when two or more
consecutive stack frames originate from the same file).
The new `MemorySourceAccessor` rather than being a slightly lossy flat
map is a complete in-memory model of file system objects.
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
MemoryInputAccessor is an in-memory virtual filesystem that returns
files like <nix/fetchurl.nix>. This removes the need for special hacks
to handle those files.
