That is, the TargetPackage instead of having a single PackageId contains
a [PackageId].

Ultimately this will allow us to support multiple .cabal files in a
single directory. But the real reason to do this generalisation now is
that it helps with the TargetImplicitCwd case. For the implicit CWD case
we need to be able to parse the target whether or not there is a package
in the CWD. So the simplest solution is to pass in all the local CWD
packages (though typically only 0 or 1) and put all of them in. Then at
the end we can check if in fact there were 0 and fail.

When we do want to support multiple .cabal files in a dir, we'll also
need to adjust the project config code, and extend the syntax slightly
so that we render as the package location for the case of multiple
packages.