mp: p2_version is only properly caught for parent modules

[CarolineM]

It seems that the p2_version is caught correctly sometimes in child modules and other times it is null, but the cause is still unknown.

The generation of this version is done by concatenating the unqualifiedVersion and buildQualifier project properties. These properties are created by some part of the Tycho P2 version calculation, but it is currently unclear which step of the Tycho build populates these variables. If we figure this out, we should be able to identify why it is not running.