Use term "package"

"unit" is sort of a GHC-specific term. Head.hackage is definitely
building Cabal packages so we can avoid some confusion by just calling
them "packages". This is probably easier for users to understand.

Some discussion about this came during Zurihac 2023. For instance,
https://youtu.be/XfTinQPjDQw?t=2774 where SPJ suggests a more distinct
split between "units" and "packages". It also came up while discussing
head.hackage itself, but I don't remember where or if it was recorded.