Mark output folders as "Derived".

Derived folders won't appear in "Open Resource", polluting
names with classfiles.

Fixes #1000260 (which regressed at some point in time).(cherry picked from commit f97085fe328824504bc15fcd5f32b20f69d18082)
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(cherry picked from commit 8eeb650f70f68d3804cf93e6ad07ae30bbff7e3c)

Conflicts:

	org.scala-ide.sdt.core/src/scala/tools/eclipse/buildmanager/sbtintegration/EclipseSbtBuildManager.scala
Merge pull request #559 from dotta/backport/518-and-533

Backport/518 and 533