Fix `pex3 lock export` handling of exotic reqs. #2643

Job	Run time
Check GitHub Organization	1s
tox -e py312-pip24_0	18m 15s
tox -e py312-pip24_0-integration	39m 22s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	5m 52s
./dtox.sh -e py27-pip20	11m 58s
./dtox.sh -e py37-pip20	15m 53s
./dtox.sh -e py311-pip20	14m 39s
./dtox.sh -e py311-pip22_3_1	15m 23s
./dtox.sh -e py311-pip23_1_2	15m 15s
./dtox.sh -e py312-pip24_0	14m 49s
./dtox.sh -e py313-pip24_0_patched	16m 22s
./dtox.sh -e pypy310-pip20	24m 19s
./dtox.sh -e pypy310-pip22_3_1	23m 41s
./dtox.sh -e pypy310-pip23_1_2	23m 7s
./dtox.sh -e py27-pip20-integration	20m 50s
./dtox.sh -e py37-pip22_3_1-integration	44m 42s
./dtox.sh -e py37-pip23_1_2-integration	43m 52s
./dtox.sh -e py311-pip20-integration	24m 47s
./dtox.sh -e py311-pip22_3_1-integration	29m 23s
./dtox.sh -e py311-pip23_1_2-integration	27m 16s
./dtox.sh -e py312-pip24_0-integration	35m 26s
./dtox.sh -e py313-pip24_0_patched-integration	39m 38s
./dtox.sh -e pypy310-pip20-integration	41m 54s
./dtox.sh -e pypy310-pip22_3_1-integration	54m 13s
./dtox.sh -e pypy310-pip23_1_2-integration	48m 4s
Gather Final Status	1s
	10h 49m 2s

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