Small UI change.
Change implementation of VsSolution.SearchInstalled().

Old implementation is getting packages from PackageManager.LocalRepository.
However, this doesn't work when there are INuGetPackageManager projects. The right
way is to search each project's InstalledPackages.
PackageManagerControl.SetBusy() should use a counter so that SetBusy() calls
can be nested.
Install options are visible only when the action is not uninstall.