React to deps.json change wrt PrivateAssets

[AndriySvyryd]

dotnet/sdk#45259 removes PackageReferences and ProjectReferences with PrivateAssets="all" from the deps.json. This means that transitive EFCore.Design references will no longer be discovered by our tools.

[AndriySvyryd]

@bricelam If that change does go through, I'm not sure that there's much that we can do other than documenting that EFCore.Design should be referenced directly from every project that would be targeted by tools.

[bricelam]

Using AssemblyLoadContext (mentioned in #18840) would be an alternative to calling dotnet exec with the project's .deps.json file. This would allow us (Edit: lol, I can't help myself; it just comes out. I mean you guys) to define where it gets loaded from.