mvn4 aggregator branches

[Guillaume Nodet <gn...@apache.org>]

I've set up an overall aggregator and set up branches named `mvn4` to align
the projects together in order to have something that is easier to work
with.

git clone https://github.com/apache/maven-sources.git
cd maven-sources
git checkout mvn4
git submodule update
mvnd install -DskipTests
-Dinvoker.skip -Dpgpverify.skip -Darchetype.test.skip

You should also be able to load the whole tree inside your IDE.

All dependencies are not aligned, but the ones from maven should (or
almost, I may have missed some bits).

The goal is to be able to easily track api usages and see how to proceed to
clean things a bit for maven4.

There's already a bunch of partial migration that has been done in order to
align the versions. And those may need to become real PRs at some point.

-- 
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Guillaume Nodet