Confine Plexus as much as possible, switch to Lookup

Confine plexus use in core as much as possible. Probably almost all uses may be replaced with API Lookup.

Cleanup unused members as well, I found at least two components:

• org/apache/maven/lifecycle/DefaultLifecycleExecutor
• org/apache/maven/lifecycle/internal/LifecycleStarter

but I bet there are more...

Given clients should inject and not manually instantiate components, ctor changes should be hidden from them (managed by DI). Hence, changes like these are "safe" as long as client code "plays by the rules".