frequent package rebuilds (slow and wasteful)

[ongardie]

Currently, packages are rebuilt often for no good reason. They don't really have a sense of when their inputs changed meaningfully. There's no distinction between:

• packaging up the same exact thing,
• packaging up a thing that's equivalent but has a fresher cache,
• packaging up a new thing that's really not that different for downstream, and
• packaging up a new thing that's critically different for downstream (like a security fix).

They also have no good way of maintaining caches when their inputs do change, so they redo most or all of their work when that happens.