[Core] Always include External in Mac pkgconfig search path
[Core] Fix only scanning global framework directories once
[Core] Minor cleanup in MonoDevelop.Core.Assemblies
[Core] Fix detecting assembly target fx via TargetFrameworkAttribute

IKVM by default falls back to lazy-loading assemblies via reflection,
which was failing, but we didn't need those anyway. Plug in a resolver
that resolves every reference as a "missing assembly".