defer plugin dependency resolution to execution rather than at addPlugin

because plugins are established at the start of the run, dependencies are all resolved at the start, even if some are never executed. This means getting dependencies of mojos you aren't even using. This is something to improve in the lifecycle executor, but it would still be good to defer plugin dep resolution until execution, the same as it is for the project itself. It would also mean not getting such a large dep hit at the start of a reactor build.

This would complicate the set up of the classworld, otherwise it would be a quick fix.