[Package Validation] Add support for multiple assemblies in the package.

[Anipik]

Currently package validation support only one assembly in the package, there could be multiple copies but it would be the same dll.

But some packages like Microsoft.ML contatins more than one assembly.

cc @safern @ViktorHofer @ericstj

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ericstj]

What happens with this today? Might make sense to take a brief look and put an error message in place if we can't implement it yet.