Speed up reprepro import and stage manipulation #399

Job	Run time
sanity	14s
unittests	10s
focal, broken, cont-on-err / build debs	3m 48s
focal, broken / build debs	2m 47s
bookworm, good, verbose=bloom / build debs	4m 0s
jammy, good, verbose=true / build debs	3m 34s
jammy, good, timeout=3 / build debs	3m 16s
jammy push / deploy	6s
bookworm squash / deploy	10s
	18m 5s

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