Verify that the new setParentServices()'s lookupFromScope(..., EXPLICIT) behavior is what is expected

[Zhuinden]

Currently, all service lookup done to the parent services (added in 9019cea ) is done with ALL, even if you passed EXPLICIT.

On one hand, passing EXPLICIT up would make sense, as that's what you received.

On the other hand, knowing that you can't pass up ALL would mean your child stack knows a bit too much about what scopes are available in the parent, whom you should be treating as a black box.

I can technically argue for both sides, so I made it unified and it just does a "lookup from provided scope", where the default has always been ALL.