Correctly build projects that depend through exported dependencies.

The Scala builder optimised the case where the builder was triggered with
no changes in a project, by skipping the call to the underlying Sbt builder.
This is wrong when the project has no changes itself, but a dependent project has.

Fixed #1001904.

Consider for back porting to 3.0.x
Merge pull request #553 from dragos/issue/builder-transitive-rebuilds-1001904

Correctly build projects that depend through exported dependencies.