Rename intenral code to 'FindUsages' so it better reflects that it can present results for more than just FindReferences.

Our references were previously a mix of preview 4 and some build between preview 4 and 5.  That was causing build output double writes.

When breaking up our unit test assemblies I accidentally moved some test code to a non-unit test project.  Split the file up and moved the code back to the correct test project.