Mark public libs as an experimental feature

Cabal/Cabal.cabal

Cabal/Distribution/InstalledPackageInfo.hs

Cabal/Distribution/Parsec/Warning.hs

Cabal/Distribution/Types/Dependency.hs

Cabal/Distribution/Types/LibraryVisibility.hs

Cabal/tests/CheckTests.hs

Cabal/tests/ParserTests/regressions/issue-5846.format

...l/tests/ParserTests/regressions/issue-6083-pkg-pkg.format

Cabal/tests/ParserTests/regressions/public-multilib-1.cabal

Cabal/tests/ParserTests/regressions/public-multilib-1.check

Cabal/tests/ParserTests/regressions/public-multilib-2.cabal

Cabal/tests/ParserTests/regressions/public-multilib-2.check

cabal-testsuite/PackageTests/MultipleLibraries/cabal.out