Uniformly treating native-inputs in native or cross build contexts

[Maxim Cournoyer]

Hello Guix,

In #60857, I've unified the cross/standard builders for the
pyproject-build-system; even their bags representation are now
shared. It enables fixing things such as #25235.

Going forward, I think it'd be beneficial to apply the same strategy to
other build systems, for consistency and to allow filtering purely build
inputs from the inputs captured in the wrap phases.

Thoughts, concerns?

-- 
Thanks,
Maxim