Use cargo read-manifest instead of metadata

The `read-manifest` json output is a subset of `metadata` for the
current `Cargo.toml` only, even tighter than `--no-deps` provides.

rust2rpm/metadata.py

@@ -139,7 +139,7 @@ class Metadata(object):
     def from_json(cls, metadata):
         self = cls()
 
-        md = metadata["packages"][0]
+        md = metadata
         self.name = md["name"]
         self.license = md["license"]
         self.license_file = md["license_file"]
@@ -171,8 +171,7 @@ class Metadata(object):
     @classmethod
     def from_file(cls, path):
         do_decode = sys.version_info < (3, 6)
-        # --no-deps is to disable recursive scanning of deps
-        metadata = subprocess.check_output(["cargo", "metadata", "--no-deps",
+        metadata = subprocess.check_output(["cargo", "read-manifest",
                                             "--manifest-path={}".format(path)],
                                            universal_newlines=do_decode)
         return cls.from_json(json.loads(metadata))