Sanitize paths that are based on Unity Product Name

[dkoozer]

Our Unity project's Product Name field contains a colon, which is apparently being used when generating one of the .sln files. We get the following error when using the MSBuild/Regenerate C# SDK Projects menu item:

DirectoryNotFoundException: Could not find a part of the path "C:\ProjectFolder\Assets\GameTitle : GameSubtitle.msb4u.sln"

I am on Windows 10, using Unity 2018.4.5f1 with MSBuildForUnity 0.8.3.