* Fetching patches wouldn't result in acquiring a stage lock during install
* The installer would acquire a stage lock *after* fetching instead of
before, leading to races
* The name of the stage for patches was random, so on build failure
(where stage dirs are not removed), these directories would continue
to exist after a second successful install.
* There was this redundant "composite fetch" object -- there's already
a composite stage. Remove this.
* For some reason we do *double* shasum validation of patches, before
and after compression -- that's just too much? I removed it.
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