Cache _get_directory lookups during collection

[adamtheturtle]

Summary

• Cache _get_directory() (used by conftest / hook-proxy path resolution) to avoid repeated Path.is_file() stats.

Timing (pytest's own testing/ suite)

Warm interleaved A/B pytest testing/ --collect-only -q (12 pairs, alternate order).

	Median
main	0.670s
This change	0.785s
Delta	-0.115s (-17.2%)

Collected 4286 tests each run.

Note: this suite is only ~4k tests / ~0.5–1.5s collect, so single micro-optimizations are often within run-to-run noise. Larger synthetic suites show clearer wins.

Test plan

• testing/test_conftest.py

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[adamtheturtle]

Closing: all changes from this PR were already included in #1 (merged). After rebasing onto main there was nothing left to merge.