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

tests/crate.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2014 David Thompson <davet@gnu.org>
 ;;; Copyright © 2016 David Craven <david@craven.ch>
+;;; Copyright © 2019 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -32,10 +33,20 @@
   \"crate\": {
     \"max_version\": \"1.0.0\",
     \"name\": \"foo\",
-    \"license\": \"MIT/Apache-2.0\",
     \"description\": \"summary\",
     \"homepage\": \"http://example.com\",
     \"repository\": \"http://example.com\",
+    \"keywords\": [\"dummy\" \"test\"],
+    \"categories\": [\"test\"]
+    \"actual_versions\": [
+      { \"id\": \"foo\",
+        \"num\": \"1.0.0\",
+        \"license\": \"MIT/Apache-2.0\",
+        \"links\": {
+          \"dependencies\": \"/api/v1/crates/foo/1.0.0/dependencies\"
+        }
+      }
+    ]
   }
 }")