Fix site-packages identification. (#2262) #2260

Job	Run time
Check GitHub Organization	4s
./dtox.sh -e py27-pip20	18m 19s
tox -e py312-pip23_2	26m 4s
tox -e format-check,typecheck,vendor-check,package	5m 46s
./dtox.sh -e py35-pip20	19m 42s
tox -e py312-pip23_2-integration	42m 51s
./dtox.sh -e py311-pip20	20m 22s
./dtox.sh -e py311-pip22_3_1	21m 22s
./dtox.sh -e py311-pip23_1_2	25m 18s
./dtox.sh -e py312-pip23_2	19m 4s
./dtox.sh -e pypy310-pip20	39m 21s
./dtox.sh -e pypy310-pip22_3_1	45m 44s
./dtox.sh -e pypy310-pip23_1_2	41m 12s
./dtox.sh -e py27-pip20-integration	32m 24s
./dtox.sh -e py37-pip22_3_1-integration	1h 51m 44s
./dtox.sh -e py37-pip23_1_2-integration	1h 49m 51s
./dtox.sh -e py311-pip20-integration	42m 36s
./dtox.sh -e py311-pip22_3_1-integration	1h 0m 14s
./dtox.sh -e py311-pip23_1_2-integration	53m 49s
./dtox.sh -e py312-pip23_2-integration	1h 5m 2s
./dtox.sh -e pypy310-pip20-integration	1h 20m 53s
./dtox.sh -e pypy310-pip22_3_1-integration	1h 21m 24s
./dtox.sh -e pypy310-pip23_1_2-integration	1h 18m 58s
Gather Final Status	2s
	17h 22m 6s

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