Reorganize source tree to have a "cubicweb" top-level package
Basically:
mkdir cubicweb
hg mv *.py -X setup.py cubicweb
hg mv dataimport devtools entities etwist ext hooks i18n misc schemas server skeleton sobjects test web wsgi cubicweb
Other changes:
* adjust path to cubicweb-ctl in devtools tests
* update setup.py to avoid importing __pkginfo__ (exec it instead),
replace os.path.walk by os.walk and prepend `modname` here and there
* update tox.ini to account for new test locations
* update doc/conf.py so that it still finds __pkginfo__.py and CWDIR in
doc/Makefile