Create item for all projects in solution

[rainersigwald]

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 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 NuGet/Home#4578 (comment)

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