Deal with dependency on scala.tools.nsc code

[dwijnand]

Breaking off from #191 the long term enhancements.

scala.tools.nsc doesn't guarantee backwards binary compatibility. So we can either:

• Rewrite the parts of MiMa that depend on scala.tools.nsc, or
• Consider fully cross-building MiMa.

[dwijnand]

Fully cross-building is probably an option, given this is mostly consumed via the sbt plugin.

With a minor bump, so it's a little easier to remember.

[dwijnand]

Oh, this would mean fresh MiMA releases for every Scala release...

Hmm. @SethTisue, worth it? Can this be part of the Scala release procedure?

[SethTisue]

I think that for a new Scala minor release to break MiMa is a fairly unusual occurrence. If and when it happens, we can just roll a new MiMa release at that point, that seems sufficient to me.

[dwijnand]

Reverted: scala/community-build@5816cdb

[dwijnand]

Too hard. Hopefully, we don't shoot ourselves in the foot. And if we do we'll have good reason to smash this back open and fix it properly.