Now that SourcePath uses a SourceAccessor instead of an InputAccessor,
we can use it in function signatures instead of passing a
SourceAccessor and CanonPath separately.
This also reworks the Mercurial fetcher (which was still using the
old cache interface) to have two distinct cache mappings:
* A ref-to-rev mapping, which is store-independent.
* A rev-to-store-path mapping.
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).
