rename scala-mode2 to scala-mode

[fommil]

requires creating two branches, scala-mode containing the new code and scala-mode2 depending on scala-mode, then updating MELPA to point at the branches.

Then after a week passes, we can move scala-mode to master and tag a stable release, and update scala-mode2 to also give a user warning on load to force everyone to migrate.

Just boring release management basically. To be done on a rainy day.