Propagate Unity context to external projects

[olga-microsoft]

Propagate Unity context to external projects. My dependency references UnityEditor and when the dll is copied to the output folder, MSBuild breaks, so I have to manually remove it.

[andreiborodin]

This is similar to the #67 issues, however, in that case the source lives outside of the Unity project but the C# project file has direct references to the current Unity project's generated files.

In this case, the C# project file has no references to any of the generated files of the current Unity project, but must be given the appropriate references to build.

[andreiborodin]

This is also similar to having one Unity project references projects generated in another Unity project but using the former's data.