Use separate representations for canonical repository vs. commit (#317)
Given `https://github.com/pypa/package.git#subdirectory=pkg_a` and
`https://github.com/pypa/package.git#subdirectory=pkg_b`, we want these
to map to the same shared _resource_ (for locking and cloning), but
different _packages_ (for determining whether the wheel already exists
in the cache). As such, we need two distinct concepts for "canonical
equality".
Closes #316.