Split reactor POM into separate parent and reactor POMs

Currently the POM in the root of the project is used both as reactor (or aggregator) and parent for the Maven modules. That's done in many projects, but leads to a minor annoyance: All plugins listed in this POM are executed for all modules and the aggregator itself, it's not possible to have a plugin in the aggregator ("at the end") only. By splitting the parent POM out into a separate file that would be possible, and we could avoid such things like the newly introduced profile with file existence check from #615.

Does anyone see a reason why NOT to split out the parent POM content into a separate file? E.g. moving it into _reposityroot_/parent/pom.xml? Maven will be happy with both, but I don't want to break anything else on CI that I might not be aware of.