Fix for sbt#1648 - warn when no or multiple main classes
Remove DefDef and check rhs

- lhs pattern matching occurs only for vals.
- def also breaks due to proc notation.
- instead of pretty printing, we can just check rhs.
Fixes #1684. cached resolution applies overrides on conflict

This change brings over dependency overrides to artificial graph.
However, it seems forced might win, so I need to take overrides logic
in account during conflict resolution.
Merge pull request #1686 from sbt/wip/fix-1684

Fixes #1684. cached resolution applies overrides on conflict
Merge pull request #1669 from kretes/1648-warn-about-main-classes

Fix for https://github.com/sbt/sbt/issues/1648 - warn about multiple or no main classes
Bump version for 0.13.7-RC1
Add warning about incompatible parser.
Fix markdown for code example.
Fix for sbt#1180 - discovered classes are now sorted