Inline scala-{library, backend} submodules

[OlivierBlanvillain]

This PR replaces the scala2-library submodule by copy pasting the relevant source files in tests, and does likewise for the scala-backend subodule by pulling the 16 files we use in compiler/src/scala/tools/nsc/backend/.

After this PR, the submodules will only be used for the community build, which simplifies the build, the setup instructions and hopefully everyone's live when checking out branches. The only downside is that this doesn't preserve the git history of these 16 backend files, one will have to follow the link in the commit message to get access to the full history.

[Blaisorblade]

👍 on avoiding submodules most of the time!

FWIW, if you ever want git support for moving changes upstream, maybe git subtree is useful (I use it myself). It copy-pastes the files, so 99% of the users have to learn nothing about submodules or subtree, but also allows to pull or push changes to your local copy to the upstream repo (where you can browse the history). Pulling is fast, tho pushing is too slow IME.