import: crate: Separate crates.io API from actual conversion.

import: crate: Separate crates.io API from actual conversion.

This provides a clean separation between bindings to the
https://crates.io/api/v1 API and actual conversion to Guix package
sexps.

As a side-effect, it fixes things like "guix import blake2-rfc", "guix
refresh -t crates", etc.

* guix/import/crate.scm (<crate>, <crate-version>, <crate-dependency>):
New record types.
(lookup-crate, crate-version-dependencies): New procedures.
(crate-fetch): Remove.
(crate->guix-package): Rewrite to use the new API.
(latest-release): Likewise.
* guix/build-system/cargo.scm (%crate-base-url): New variable.
* tests/crate.scm (test-crate): Update accordingly.

fixlet