Fix NuGet packages showing as installed in solution.

NuGet packages that were still in the packages folder but
not installed in any projects are now not shown in the
Installed tab when managing NuGet packages for the solution.
Fix old NuGet packages not being removed on updating.

Updating to a new NuGet package or installing a new NuGet
package with the old one still installed would leave the
old unused NuGet package in the packages folder.