celery: move Whoosh indexing task to whoosh library where it belongs Avoid bundling everything from many different layers in one big task library. This is more feasible now when we don't need kallithea.CELERY_APP set at import time.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.