Don't skip over imports and other nodes containing nested statements in import collector

[MichaReiser]

Summary

This is a follow up for #13441

enter_node is called for every node and it's important that it returns true for any node between ModModule and an import statement, including the import statement itself.

Getting this right in enter_node seems hard and it's easy to forget adding a new "include" when a new compound statement gets added.

This PR moves the traversal logic into the visist_stmt by requiring explicit match arms for each statement. I think this is easier to understand
and less error prone.

Also... enter_node doesn't get call when a visitor overrides visit_stmt. That's an oversight from my side when designing the visitor.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

Let me add a test for this