Per-project rules assembly csprojen.
Previously: all .build.cs and .target.cs were being grouped into a single project, which resulted in duplicate symbols when trying to build the project.
Now: multiple assemblies csprojs are generated, one for each Engine & Game project. Also, includes preprocessor defines.
#jira none
#rb ben.marsh
[CL 16658600 by jonathan adamczewski in ue5-main branch]
8d1ebef30a