Verify fixtures have sources (and mention same-assembly requirement for xunit/xunit#2311)

[etherfield]

Resolves 2311

[bradwilson]

@etherfield Looking over this source and understanding what's going on here, it occurs to me what's really being done here: verifying that fixtures can be provided at runtime. The need for the collection definition to be in the same assembly is a related but orthogonal requirement which can be surfaced during this discovery. It occurred to me as I realized that this code needed to be updated for IClassFixture<> support what it was really doing.

Since this is about 80% of the way there, I'm going to make the pivot and adapt it to do the primary task of ensuring fixture sources are specified with the secondary task to letting people know when their fixture sources can't be found that it may be caused by the fact that their collection definition lives in another assembly.