Element state handling fixes

Fixes #202 (closed) and additional issues with non-strict mode as described in the commit messages. We need to increase test coverage.

added 2 commits

• 88187f12 - tests/frontend/pull.py: Add pull test for non-strict build plan
• 043d56a6 - tests/frontend/pull.py: Add track+pull test for non-strict build plan

Compare with previous version

Added two test cases for non-strict build plans. The first test case covers pulling with the weak cache key (already succeeds in master) and the second test case covers #202 (closed) (fails in master, succeeds with this branch).

enabled an automatic merge when the pipeline for 043d56a6 succeeds

merged