Support a new mode for P2 target locations to include as much as possible

[laeubi]

Currently a P2 Location has only two modes:

1. planner starting from the root IUs everything must resolve without any conflicts
2. slicer only the root IUs are included

What often is missing would be a mode that includes "as much as possible" without performing a planner operation, just starting at the root IUs and traverse down to collect whatever can be found.

[merks]

So like the transitive closure but without failure when the closure is not complete.

Oomph tries to do this (actually does this) this for targlets basically by taking all the metadata and transforming all requirements into optional greedy requirements and then using the planner which as a result of optional cannot fail and because of greedy resolves as much as possible...

[laeubi]

Yes, what is used under the hood is not something a user would need to care, it should just help in some setup where I want to include something that is incomplete (from a planner point of view) and I still want to use some of the things can still bring in value.