Downgrade NoLibraryFound from an error to a warning

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

(cherry picked from commit 312a4124)

# Conflicts:
#	Cabal/src/Distribution/Simple/Install.hs