rcm: introduce rpmmd member of the api structure

This is needed for unit tests, because it wasn't possible to mock the
rpmmd module before. This also requires that the checksum is moved to
the compose request and evaluated in the endpoint handler instead of
push compose. I think it makes sense to have the checksum in the compose
request directly.

Also a "module platform ID" is required now, but we don't have the
"global" distribution any more, so this patch introduces mapping from a
distribution to the module platform ID.

cmd/osbuild-composer/main.go

@@ -72,7 +72,7 @@ func main() {
 	// Optionally run RCM API as well as Weldr API
 	if len(listeners) == 3 {
 		rcmListener := listeners[2]
-		rcmAPI := rcm.New(logger, store)
+		rcmAPI := rcm.New(logger, store, rpmmd.NewRPMMD())
 		go rcmAPI.Serve(rcmListener)
 	}