get doc gen working post-backend-refactor

'goal builddict' runs instead of crashing, yay.
re-enabled builddict tests; took out the "len(usual_symbols)>20" test; couldn't figure out an easy way to get the "usual symbols". No big loss; I don't think that test ever found an error.

Added a hardwired entry for the now-gone "egg" in BUILD dictionary: "If your pants is old, there might be `egg` in there." Sounds like instructions for someone wearing an outfit from the Owl Thrift Shop, but there you go.

gen.py grabbed code from "tasks" and "targets" dir. But now there's more than one of those dirs, so check 'em all.

Testing Done:
./pants tests/python/pants_test:all

./build-support/bin/publish_docs.sh -o
  compare list of entries in an old and new BUILD Dictionary (and thus notice "egg" went bye-bye)

Reviewed at https://rbcommons.com/s/twitter/r/463/