Factor out a `LockDownloader`. #3219

Job	Run time
Check GitHub Organization	1s
./dtox.sh -e py27-pip20	9m 55s
tox -e format-check,lint-check,typecheck,enum-check,vendor-check,package,docs	10m 56s
tox -e py313-pip24_3_1	12m 45s
./dtox.sh -e py38-pip22_3_1	18m 33s
tox -e py313-pip24_3_1-integration --shard 1/2	32m 14s
./dtox.sh -e py311-pip23_3_2	18m 21s
tox -e py313-pip24_3_1-integration --shard 2/2	25m 18s
./dtox.sh -e py313-pip24_3_1	15m 30s
./dtox.sh -e py314-pip24_3_1	14m 54s
./dtox.sh -e pypy310-pip24_3_1	27m 45s
./dtox.sh -e py27-pip20-integration	28m 23s
./dtox.sh -e py38-pip22_3_1-integration --shard 1/2	31m 49s
./dtox.sh -e py38-pip22_3_1-integration --shard 2/2	28m 8s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	29m 23s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	30m 8s
./dtox.sh -e py313-pip24_3_1-integration --shard 1/2	26m 23s
./dtox.sh -e py313-pip24_3_1-integration --shard 2/2	27m 3s
./dtox.sh -e py314-pip24_3_1-integration --shard 1/2	26m 51s
./dtox.sh -e py314-pip24_3_1-integration --shard 2/2	28m 51s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 1/2	46m 2s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 2/2	49m 30s
Gather Final Status	1s
	8h 58m 44s

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