Follow-up to my own #3171. That PR gave the deploy path its own cache and
left the paragraph above it saying "package builds and checks reuse this"
— which stopped being true in the same commit that made it false.
Merged into one accurate lead-in. The surviving facts (built once, every
consumer passes the same `cleanSrc`) are still worth stating; the claim
about who consumes it belongs to the two blocks below, which say it
correctly.
feeabe97af