Fix absolute path and cyclic import

[frademacher]

Thanks a lot for the PR, which I am happy to merge when my comments got fixed.

Please also check the following:

• Make sure that a line width of 100 columns is not exceeded.
• Please adapt the commit message LEMMA Utils: changed relative paths to absolute paths according to the style guide (https://seelabfhdo.github.io/lemma-docs/developer-guide/coding-style/#commits) to something like LEMMA Utils: Use absolute paths instead of relative paths for checking the existence of imported files.
• Similarly, please adapt the commit message LEMMA utils: Changed the cyclic validation... to something like Data DSL: Fix infinite loop in cyclic import detection (the fix really concerns the Data DSL and not LEMMA Utils, please see my comments for the detailed rationale on this comment).

[frademacher]

Thanks a lot for the revised (and greatly improved) PR! I left only a few minor remarks. In addition to these, please also remove the redundant merge commit (https://github.com/SeelabFhdo/lemma/pull/47/commits/0578e3558767fa3c80f1f1f97c51a93355364d57).