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

This is a backport of PR #2970. In order to move the build to Scala OSGi bundles, we need to be able to build both on 2.10 and 2.11.

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).