Convert private lib in common stanza

[jneira]

• To workaround the issues involving the stack based cradle: stack cradle fails to load private lib #114 and Failing to use haskell-language-server on itself #121
• I've not changed the build info of the previous private lib, in the hope we will restore it
• I've tested it locally:
  • You can open Test.Hls.Util after building the project with test enabled stack build --test --no-run.tests
  • When opening exe/Main.hs after deleting .stack-work, it hits Error using stack explicit cradle: No prefixes matched for Paths_haskell_language_server.hs mpickering/ghcide#50 but it seems it does not need a stack build(it starts the initial ghc session)

[fendor]

LGTM! Thank you!