Move (back) over to MSBuild solution parsing API

[DustinCampbell]

Fixes dotnet/vscode-csharp#1580

This also adds initial tests for #863.

[filipw]

great - out of curiosity, what was the original reason for the custom parsing code?

[DustinCampbell]

There are a couple of things:

1. Before, we were using the "project type GUID" to detect Unity projects and such. But I've now handled that through other means. Checking for ".csproj" is good enough.

2. When I originally tried to use MSBuild's API, I tried to use the "AbsolutePath" property for each project and found that had a bug with non-Windows slashes. Sticking with the "RelativePath" found in the solution file and keeping our existing logic for converting slashes avoids that bug.

[filipw]

thanks

[david-driscoll]

[DustinCampbell]

Thanks guys!