In the draw module, it's not easy to identify what its header is, and
where the shared functions are.
So move `draw_cache_extract_mesh_extractors.c` and
`draw_cache_extract_mesh_private.h` to the same folder as the extractors
and rename these files to make them more identifiable.
Reviewed By: jbakker
Differential Revision: https://developer.blender.org/D11991
