Define ARTEFACTS_DIR in the __init__.py at package source

[barhomi]

Context
The artefact directory is defined at the package's root currently. the ARTEFACT_DIR variable gets redefined in each unit test file. We may want to move it a tmp directory instead, probably use the same convention with the manifestlookup classes (same target directory).

Acceptance criteria

• there is only 1 place for ARTEFACT definition (in the init.py of the package)
• the definition is imported by all unit tests
• make the artefact directory live in /tmp instead of package root
• write this in the README so user can choose between /tmp/ or local