Added PipelineBuildEvent.Extension.
Added "Cleaning" log message.
Add error if the source file does not exist.
We now write a log of all the source files processed.
We can now properly clean files that have been removed from the previous build.
Added /incremental flag to disabling cleaning of previous files.
We now remove duplicate input source files.
Added SourceFileCollection.cs to MGCB for Linux and Mac.
Fixed debug config in Linux and Mac csproj.
Added target /platform, /profile, and /config options to MGCB.