Currently linker only recognizes embedded descriptors if they're called AssemblyName.xml. This is rather confusing as there's nothing in the name of the resource which describes what it's for.

We should also recognize embedded resources called ILLink.Descriptors.xml (no assembly name in them). This will also unify the handling with existing ILLink.Substitutions.xml and the coming ILLink.LinkAttributes.xml.

It's probably not worth the trouble to try to remove the existing behavior, so linker should still recognize AssemblyName.xml as well.