PM UI: improve perf of PackageSolutionDetailControlModel.GetConstraintsForSelectedProjectsAsync(...)

[dtivel]

PackageSolutionDetailControlModel.CreateVersionsAsync(...) fetches project metadata for each selected project one at a time --- twice! here and here.

When 1 project is selected, the potential for performance improvement is minimal. However, when many projects are selected the potential for improvement is great, especially in VS OE scenarios where project metadata retrieval is over the network.

At a minimum, the results of one retrieval should be reused. Ideally, project metadata retrieval can be completely avoided if ProjectVersionConstraint.ProjectName is changed to ProjectVersionConstraint.ProjectId.