Extend assembly loading warning to check for non-MSBuild but implicit dependencies like S.T.J and the SLNX library

[baronfel]

The warning/checks that tell users when there is ambiguous assembly loading is a great safety net, but it doesn't help users with dependencies that are implicitly provided by the MSbuild host - for example System.Text.Json and Microsoft.VisualStudio.SolutionPersistence. We should consider expanding our check here to see if we can help users understand when they are (incorrectly) attempting to load dependencies that the host has already loaded.