Warn if collection spends >5s traversing a directory without collecting any tests

[Zac-HD]

I recently saw #5851 (comment), which made me wonder if we could automatically detect similar cases and suggest using the --ignore flag or norecursedirs config option.

Tracking the time spent in collection on a per-directory basis and whether any test files were collected should be pretty straightforward, and from there it's just the reporting code. Specifically: depth-first search over the tree, don't recurse into children if the current node triggered the warning or took <= 5s. Alternatively, maybe this could be integrated with the collection code so that we don't need a separate step.

[Pierre-Sassoulas]

Kinda related, but maybe when the --durations option is used, we also display the collection time per directory ?