411664 Only honor "includeSource" in target files if enabled

- In the main use cases of automated builds, the includeSource attribute
  is not needed: If sources shall be delivered, one would include entire
  source or SDK features into the p2 repository. These they would need
  to be explicitly selected in the target file. If no sources shall be
  delivered, one also doesn't need the source bundles in the target
  platform.
- In order to make Tycho imitate the PDE includeSource behaviour, one
  needs to set targetDefinitionIncludeSource="honor" in the
  target-platform-configuration. Alternatively, includeSource can be
  forced to true for the Tycho build with targetDefinitionIncludeSource
  ="force".

Change-Id: I78ffcbd7b46d7dd70383ad2af335b93c709a7389
Signed-off-by: Tobias Oberlies <tobias.oberlies@sap.com>
Also-by: Mickael Istria <mistria@redhat.com>