delete "relative" svn:externals dirs
Also add autogen.sh logic to copy from authoritative sources to the correct "externals". So "src/pm/hydra/confdb" is now copied (in a special fashion) from "confdb" at autogen.sh-time. Prior to this commit these externals were simply tracked in the git repository as a consequence of their (non-trivial) export from SVN. Non-relative svn:externals, such as "src/openpa" are not dealt with in this commit. No reviewer.