Disable pattern matching in val syntax in sbt 0.13.7.

Fixes #1661 or at least works around it for now.
merged #1662
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.