Use CPSPackageReferenceNuGetProject only when both CPS and PR capabilities are set

[zivkan]

Bug

Fixes: NuGet/Home#10169
Regression: Yes (but didn't ship)

• Last working version:
• How are we preventing it in future:

Fix

Details: Add new capability check just for PackageReferences, and improve xmldoc comments for existing capability checks to explain what they do and do not tell us.

Testing/Validation

Tests Added: No
Reason for not adding tests: I'm still looking into how to test this. I'm not sure if we can mock the VS project adapter, but having written this I think we can, which means I probably need to modify this PR to add tests.
Validation:

[nkolev92]

👏

Can we add an sfproj test so we don't regress this in the future? :D
In the interest of getting it merged earlier, I think it can be done in a future PR.

[nkolev92]

LGTM :)

Still targetting P1 QB mode?

[zivkan]

I was too slow to post this before you reviewed again.

I think it might be difficult to create a E2E or Apex test to create a real sfproj project. Maybe I'm wrong and it's no more difficult than any other project, but when manually creating a Service Fabric project, it creates the sfproj, which is a packaging project, but the wizard also creates a service, for the sfproj to package. This second wizard screen has multiple options, so I don't know how that would work in Apex. Maybe it's easier in E2E, but I don't like E2E tests and think we should move entirely to Apex tests.

I created a unit test, although it's a bit messy to make testable. Yes, there's a risk that this test could pass, but real projects still fail. For now I think it's good enough.