After having been rebuffed T48491 over a patch to a library in extern, it seems clear that information is sorely lacking about which parts of the Blender source tree are or are not off-limits for patch submissions.
The enclosed patch
adds a README which tries to make clearer the status of the various subdirectories in extern. It lists which ones belong to the Blender Foundation, and refers the reader to external project sites for the rest, where I have been able to determine them: in some cases, I have not been able to do so, or the sites concerned no longer seem to exist.