SI-7862: MANIFEST.MF file for Scala sources

In order to be able to use published Scala jars as OSGi bundles in the
Eclipse build, Eclipse needs to match sources and binaries. That is
done by making source jars *source bundles*. This PR adds the required
manifest entries. Nothing else should be affected (file names remain
the same).
Merge pull request #2970 from dragos/improvement/source-osgi-headers

Fix SI-7862: MANIFEST.MF file for Scala sources