This is driving me a little crazy and is becoming a larger problem in
#2596 where I need to move more types (like `Upgrade` and `Reinstall`)
into this crate. Anything that's shared across our core resolver,
install, and build crates needs to be defined in this crate to avoid
cyclic dependencies. We've outgrown it being a single file with some
shared traits.
There are no behavioral changes here.
Zanie Blue