Downgrade NoLibraryFound from an error to a warning

[TeofilC]

This makes Setup copy/install succeed if there's
nothing to do because the package doesn't contain
a library or executable.

This allows downstream users of Cabal to avoid having to add workarounds for this edge case.

Resolves #6750

Let me know what you think. If this sounds OK, I'll add a test case, which test suite would be best?

---

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[Mikolaj]

LGTM, though I'm not familiar with these code paths.

Re tests, the default seems to be cabal-testsuite.

[TeofilC]

I've added a test case of a package that is just a test suite. In fact this is one of the options offered by the cabal init setup wizard

[alt-romes]

Thanks. I don't see a need to emit an error here so this seems like an improvement.

[Mikolaj]

@TeofilC: could you kindly set a merge label, confirming you finished tweaking the PR, as per https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#github-pull-request-conventions? Thank you.

[TeofilC]

Sounds good. Thanks!