logilab-common #100757 modutils.modpath_from_file function is broken [validation pending]

It makes stuff like cubicweb-ctl, when installed using debian packaged, fail like:

ERROR:cubicweb.configuration:could not import the command provider server/serverctl.py: Unable to find module for /usr/lib/python2.7/dist-packages/cubicweb/server/serverctl.py in /usr/share/cubicweb,
/usr/bin,
/usr/lib/python2.7,
/usr/lib/python2.7/plat-linux2,
/usr/lib/python2.7/lib-tk,
/usr/lib/python2.7/lib-old,
/usr/lib/python2.7/lib-dynload,
/usr/local/lib/python2.7/dist-packages,
/usr/lib/python2.7/dist-packages,
/usr/lib/python2.7/dist-packages/PIL,
/usr/lib/pymodules/python2.7
CubicWeb 3.15.1 (system mode)

This is dur to the test (line 241):

if path.startswith(EXT_LIB_DIR):
   continue

which prevent from loading /usr/lib/python2.7/dist-packages/cubicweb/server/serverctl.py, since EXT_LIB_DIR is /usr/lib/python2.7/dist-packages/