cacheprovider: `--lf` file-skipping feature doesn't work for files in packages

[bluetech]

--lf has a feature where if a certain Module (python file) does not contain any failed tests, it is skipped entirely at the collector level instead of being collected and each item skipped individually. When this happens the collection summary looks like this:

run-last-failure: rerun previous 1 failure (skipped 1 file)

However, this feature doesn't work for Modules inside of Packages, only for those directly beneath the Session.

[bluetech]

The fix in #11055 still doesn't cover all cases.

Problem

The file skipping works by keeping a set of all test files which had a failure (get_last_failed_paths), then skipping files which are not in it.

However, this skipping is only activated once at least one failure occurs, otherwise some Bad Things Happen (#6968?).

This means that if a clean file is collected before a file with known failures is collected, it won't be skipped, because the mechanism hasn't been activated yet.

So to prevent the previous item from occurring, the files are sorted such that files with known failures are collected first.

Unfortunately the sorting can only be done one collector at a time. So it is foiled by cases like this:

test_clean.py
pkg/__init__.py
pkg/test_failing.py

pkg is not sorted before test_clean.py because it is not a test file.

Proposed fix

While the whole feels like it can be done more robustly, I propose a simpler fix for now. Instead of get_last_failed_paths containing only test files (test_clean.py, pkg/test_failing.py), have it contain the directories as well (pkg). This way pkg is sorted before test_clean.py.