Split out inter-project resolver into its own Chain.

Fixes #1565

* Create two chains if we have inter-project resolver
  - One which ensures inter-project deps are always taken
  - One which will look in all "other" repositories for
    dependencies and will use the "most up-to-date" -SNAPSHOT
    strategy.
* No additional tests, as this would simple break every
  multi-project test if it were wrong.
Merge pull request #1577 from sbt/wip/fix-inter-project-chains

Split out inter-project resolver into its own Chain.