352081 Resolve references in feature.xml directly from TP

- For inclusions in the feature.xml, where versions are replaced during
  the build, directly query the target platform for the available
  versions instead of the result of dependency resolution. This avoids
  surprises with the version selection policy of the p2 resolver. We are
  now guaranteed to pick the latest version that matches the
  specification.
- Add methods to the TargetPlatform which allows to query for artifacts.
- Also include reactor artifacts in the jointArtifacts in the final
  target platform. (This is also an important step for bug 372780.)

Integration test changes:
- Make the integration test testing the reference resolution within
  the reactor harder by using references with a 'qualifier' literal and
  by setting a different qualifier in each project. This ensure that the
  respective versions are used, and that qualifier in references is not
  interpreted as the current project's qualifier (cf. bug 373817). This
  integration test also covers the feature's own version replacement
  which is no longer unit tested.
- Also: Improve assertion style in ArtifactPublishingAndAssemblyTest.
- Removed integration test for qualifiers which used the deprecated
  local resolver. feature.xml references are no longer replaced when
  this resolver is used.

Bug: 352081
Bug: 372780
CQ: 7358 (Mockito 1.8.4)
CQ: 8987 (Hamcrest core 1.3.0)
Change-Id: I1fd31841502699d5c8149223bdbfa6ded19cb0c2