Ideally m2e-wtp would disable m2eclipse-wtp when
installed, but I'm afraid it seems impossible (yeah I suck at
eclipse/OSGi stuff).
So that means the only way to install m2e-wtp for existing
m2eclipse-wtp users will be to either start from a pristine
eclipse installation or uninstall m2eclipse-wtp beforehand,
breaking the update process for products already embedding
m2eclipse-wtp.
Using p2.inf on a repository, we can add extra actions to
installations. Maybe there is the necessary stuff to uninstall an IU
if present... I honestly don't know a lot about those p2.inf files: