* guix/build/cargo: (package-cargo-inputs): Add it.
(package-cargo-development-inputs): Add it.
(crate-closure): Add it.
(expand-crate-sources): Add it.
(lower)[private-keywords]: Add #:cargo-inputs and
[bag]: Use expand-crate-sources to augment build-inputs.
Signed-off-by: Chris Marusich <cmmarusich@gmail.com>