[bug#35318] [PATCH] Update cargo-build-system to expand package inputs

[Ivan Petkov <ivanppetkov@gmail.com>]

Hi Ludo,

> On May 6, 2019, at 1:00 AM, Ludovic Courtès <ludo@gnu.org> wrote:
> 
> There’s nothing written in terms of “dependencies”; instead everything
> is written in terms of “inputs”, so I’d like to remain consistent here.

I admit, I wasn’t feeling very inspired with coming up with a name, but I wanted
to avoid confusing it with the existing package-transitive-inputs procedure.

> If we need something more specific, I’d rather see it as a private
> procedure in (guix build-system cargo-build-system).

I wanted to reuse the core traversal/memoization that was defined in the 
original package-transitive-inputs procedure, but I can fork the implementation
as a private procedure in the cargo-build-system for now.

> I’m not sure I want to know the details :-), but it seems to be what
> ‘package-transitive-inputs’ does, no?

package-transitive-inputs captures all transitive propagated-inputs, but the
cargo-build-system needs all transitive propagated-inputs and regular inputs
as well (modeling cargo dependencies as propagated-inputs does not seem
appropriate).

Thanks,
—Ivan