Automatically scan `<ProjectReference>` projects for bound Java libraries

[jpobst]

Context: #8649

Previously, we added support for Java Dependency Resolution to help users ensure that their binding projects fulfill all needed Java dependencies.

One documented "papercut" that we do not support yet is the ability to automatically determine which dependencies are fulfilled by any <ProjectReference> elements in the binding project. Instead, these must be manually specified like this:

<ProjectReference 
  Include="..\My.Other.Binding\My.Other.Binding.csproj" 
  JavaArtifact="my.other.binding:helperlib" 
  JavaVersion="1.0.0" />

We would like to automatically query each <ProjectReference> (perhaps using the <MSBuild> task?) to determine any @AndroidLibrary items that have %JavaArtifact metadata instead of asking the user to manually specify it.