Downgrade NoLibraryFound from an error to a warning (backport #9926) (#10076)

* 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

* fixup! fix conflicts

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Co-authored-by: Teo Camarasu <teo.camarasu@tracsis.com>
Co-authored-by: Artem Pelenitsyn <a.pelenitsyn@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>