Feature #2597 -- report all paths in each cycle found.

Rather than reporting the cluster of vertexes in the dependency graph, which
is interesting but not entirely informative, we now calculate and report the
paths through the graph that form cycles.

This returns the most useful information, which is the exact path that the
dependency cycle has, allowing the user to (hopefully) immediately target it
and start to work out why the cycle has formed there.

We strongly prefer short paths through the dependency graph within the cycle,
which should report the most useful loops to target first; extended loops
involving more items will show up later if they are independently created.

We also limit the number of paths reported (default: 10) to avoid overwhelming
the error report with the combinatorial explosion that can easily result
from a large scale cycle. (eg: Package => User => Package or something.)