In resolveTargetSelectors we return [TargetSelectorProblem] which
includes a case TargetSelectorNoTargets. We split this into two cases:
TargetSelectorNoTargetsInCwd and TargetSelectorNoTargetsInProject.

The latter covers the important special case when there are no targets
in the project as a whole. The former covers the same remaining cases as
the original.

This is a useful special case since it corresponds to running commands
in a directory where there is no .cabal files and that is not underneath
a project. Add a hopefully useful error message for this case.

Also adjust the integration tests covering this case, which now split
into two separate cases.