Cachito: configure relative path if source is in subdirectory

[vfreex]

No description provided.

[openshift-bot]

Build #1

GLOB sdist-make: /mnt/workspace/jenkins/working/art-tools_doozer_PR-562/setup.py
py36 create: /mnt/workspace/jenkins/working/art-tools_doozer_PR-562/.tox/py36
ERROR: invocation failed (exit code 1), logfile: /mnt/workspace/jenkins/working/art-tools_doozer_PR-562/.tox/py36/log/py36-0.log
================================== log start ===================================
ImportError: cannot import name 'NoOpFileLock'

=================================== log end ====================================
ERROR: InvocationError for command /usr/bin/python3 -m virtualenv --no-download --python /usr/bin/python3 py36 (exited with code 1)
___________________________________ summary ____________________________________
ERROR:   py36: InvocationError for command /usr/bin/python3 -m virtualenv --no-download --python /usr/bin/python3 py36 (exited with code 1)

[thiagoalessio]

/lgtm