Fixes #1699. ignore transitive force during cached resolution

When Ivy translates pom to ivy.xml, it adds force=“true”.
So when both non-Maven dependencies and Maven dependencies are mixed, Maven dependencies always wins, which is the case for scala-library dependency added by the user.
modify test that test transitive force

This test relies on force having effect transitively for cached
resolution.
added comments
Merge pull request #1700 from sbt/fix/1699

Fixes #1699. ignore transitive force during cached resolution