Also renames `_path2confmods` to `_dirpath2confmods` for clarity (it is
expected to be a dirpath in `_importconftest`).
Uses an explicit maxsize, since it appears to be only relevant for a
short period [1].
Removes the lru_cache on _getconftest_pathlist, which makes no
difference when caching _getconftestmodules, at least with the
performance test of 100x10 files (#4237).
1: https://github.com/pytest-dev/pytest/pull/4237#discussion_r228528007