Threading issues with MEF being called on async codepaths

[nkolev92]

NuGet LegacyPackageReferenceProjectProvider synchronously requests MEF along async code path - issue
NuGet requests MEF synchronously along async code path - issue
NuGet MEF part(s?) have thread dependencies in ctors - issue

//cc @rrelyea @jainaashish

[nkolev92]

@zhili1208
Just assigning it you as you've been doing the work here.
2/3 done.