swift-package-manager

Get rid of `Utility.Path` by switching the last clients of the last
methods over to other code.  This means that we are down to just the
AbsolutePath and RelativePath types (plus a misnamed struct in the
`Options` class which will soon get renamed).

Apart from renaming `AbsolutePath` to `Path` after an appropriate amount
of discussion about whether or not that is wise, this completes the path
transition.