support only using saveAs and downloadUrl

[kymppi]

Currently there's in an edge case where the plugin download silently fails in the downloadPluginRecursive if multiple plugins with only saveAs and downloadUrl are defined.

Part of the problematic code:

	mapKey := plugin.GroupID + ":" + plugin.ArtifactID

	if version, ok := pluginMap[mapKey]; ok {
		if plugin.Version != version {
			return fmt.Errorf("version conflict (%s, %s) for %s", plugin.Version, version, mapKey)
		}

		return nil
	}