handle nicely typical installation other than debian package / mercurial forest * new _find_prefix method to detect installation prefix (maybe specified using CW_INSTALL_PREFIX environment variable) * set various resource accessors according to this, cleanup on the way * remove no more necessary APYCOT_ROOT hacks

debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/etwist/ usr/lib/PY_VERSION/site-packages/cubicweb/