gnu: librsvg-next: Use cargo-build-system.

gnu: librsvg-next: Use cargo-build-system.

* gnu/packages/gnome.scm (librsvg-next)[source]: Don't use computed
source. Add snippet to remove vendored crates.
[build-system]: Switch to cargo-build-system.
[arguments]: Remove configure-flags, make-flags, imported modules. Add
modules and vendor fields. Add crates to cargo-inputs and
cargo-development-inputs. Add 'prepare-for-build phase to adjust the
source before building. Add 'gnu-configure phase to add the configure
phase from the gnu-build-system. Replace the 'build, 'check and 'install
phases from the gnu-build-system.
[native-inputs]: Remove rust-1.34, rust-1.34:cargo.
(computed-origin-method, librsvg-next-source): Remove variables.