Refactor so that instead of two constructors TargetPackage and
TargetCwdPackage we have just TargetPackage with an extra bool-like field to
distinguish the two.

This simplifies things in the consumers of TargetSelector which
typically have to treat TargetPackage and TargetCwdPackage in the same
way.

Also eliminate the list field in TargetCwdPackage. We don't yet support
multiple .cabal files in one dir, and when we do we'll do the same for
the cwd targets as for other explicit targets. The only annoying thing
here is that we need to use a dummy package info value because the
representation does not allow for not having one. The tradeoff is still
worth it in terms of less verbose consumers.

Also adjust tests.