_project.py: Include fail-on-overlap setting in cache key

Jürg Billeterrequested to merge
juerg/project-cachekey into master

Description

This branch includes the fail-on-overlap project setting in the cache key to rebuild artifacts when the setting is enabled. Otherwise BuildStream may falsely claim success (using cached artifacts) despite overlaps.

This merge request, when approved, will close issue/bug: #473 (closed)

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