`cabal v2-build` reports package "not in this project" when target spec matches no components

Steps to reproduce: in Cabal checkout, run `cabal new-build cabal-testsuite`

Expected result: Starts building `package-tests`

Actual result: Up to date. Always.

There should be some sort of sanity check where if the target selection matches zero things, we complain.

Note that cabal-testsuite has no library, and I believe specifying a package does NOT select the test suites by default. That's OK; just have to tell the user what they probably meant to say.