bitbake: fetch2/crate: add upstream latest version check function

This is actually rather easy: crate web API provides a json
with all the versions, for example:
https://crates.io/api/v1/crates/cargo-c/versions

(Bitbake rev: f6c2755db9a1f88c8534193b420fa31d135945e6)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/fetch2/crate.py

@@ -70,6 +70,7 @@ class Crate(Wget):
             host = 'crates.io/api/v1/crates'
 
         ud.url = "https://%s/%s/%s/download" % (host, name, version)
+        ud.versionsurl = "https://%s/%s/versions" % (host, name)
         ud.parm['downloadfilename'] = "%s-%s.crate" % (name, version)
         if 'name' not in ud.parm:
             ud.parm['name'] = '%s-%s' % (name, version)
@@ -139,3 +140,11 @@ class Crate(Wget):
             mdpath = os.path.join(bbpath, cratepath, mdfile)
             with open(mdpath, "w") as f:
                 json.dump(metadata, f)
+
+    def latest_versionstring(self, ud, d):
+        from functools import cmp_to_key
+        json_data = json.loads(self._fetch_index(ud.versionsurl, ud, d))
+        versions = [(0, i["num"], "") for i in json_data["versions"]]
+        versions = sorted(versions, key=cmp_to_key(bb.utils.vercmp))
+
+        return (versions[-1][1], "")