A BuildRequest describes the request of something to be run on the builder.
It is distinct from an actual [Build] that has already happened, or might be
currently ongoing.
BuildConstraints represents certain conditions that must be fulfilled
inside the build environment to be able to build this.
Constraints can be things like required architecture and minimum amount of memory.
The required input paths are not represented in here, because it
wouldn’t be hermetic enough - see the comment around inputs too.