weldr: Preload metadata at startup

For each of the supported distros start a goroutine to depsolve
'filesystem' which will preload the metadata making subsequent responses
faster.

This is safe to do without limits because we only supposed a limited
number of distros, and without additional locking because this is the
the same as hitting the API with multiple depsolve requests at the same
time.

cmd/osbuild-composer/composer.go

@@ -123,6 +123,8 @@ func (c *Composer) InitWeldr(repoPaths []string, weldrListener net.Listener,
 	}
 	c.weldrListener = weldrListener
 
+	// Preload the Metadata for all the supported distros
+	c.weldr.PreloadMetadata()
 	return nil
 }