The assembly (AsciiDoc) source should be stored on the src.contents property of the export file. This aligns with how pages are handled when Assembler is being used. (I would say that the `keep_source` key on the site-wide AsciiDoc config would have to be enabled. However, Assembler already forcefully enables that for its own needs. We might want to think about setting that temporarily and honoring the original setting. Some experimentation might be needed). This feature provides other extensions the opportunity to access the assembly source without having to look for a file in the build directory.