LOOT now uses Qt's support for Markdown, but its support for GFM is
bugged so LOOT uses CommonMark instead. The practical impact is very
minor, but reflect the difference in libloot's docs.
Skip adding an edge between a pair of plugins if all the following
conditions are met:
- the edge is to be added due to group membership
- adding the edge would introduce a cycle in the absence of any
other group membership edges.