Create item for all projects in solution

The existing solution metaproject doesn't actually have an item for every project in the solution. It has a pointer to every project, but they're not always the real project on disk--[some circumstances](https://github.com/Microsoft/msbuild/blob/99e95d0f55d2af82767ffd8c81dcfdc663092e49/src/Build/Construction/Solution/SolutionProjectGenerator.cs#L1000-L1039) cause the link to point to a virtual project with the extension `.metaproj` appended instead.

This works fine for build, but not for solution-extending build logic that wants to do something with each project.

See https://github.com/NuGet/Home/issues/4578#issuecomment-279111188

One wrinkle: the "is this MSBuild format" stuff might still be relevant.