MM-21103: change plugin signature path (#13360)

* MM-21103: change plugin signature path

Save as `<plugin_id>.tar.gz.sig` instead of `<plugin_id>.sig`. The latter was a relic of the previous design to support multiple plugin signatures, but now creates an inconsistency with how the original source files were supplied as `<some_name>.tar.gz` and `<some_name>.tar.gz.sig`.

Fixes: https://mattermost.atlassian.net/browse/MM-21103

* relax signature matches to avoid assuming signatures always exist

app/plugin.go

@@ -244,6 +244,7 @@ func (a *App) SyncPlugins() *model.AppError {
 	if appErr != nil {
 		return appErr
 	}
+
 	for _, plugin := range pluginSignaturePathMap {
 		reader, appErr := a.FileReader(plugin.path)
 		if appErr != nil {
@@ -593,6 +594,7 @@ func (a *App) getPluginsFromFolder() (map[string]*pluginSignaturePath, *model.Ap
 	if appErr != nil {
 		return nil, model.NewAppError("getPluginsFromDir", "app.plugin.sync.list_filestore.app_error", nil, appErr.Error(), http.StatusInternalServerError)
 	}
+
 	pluginSignaturePathMap := make(map[string]*pluginSignaturePath)
 	for _, path := range fileStorePaths {
 		if strings.HasSuffix(path, ".tar.gz") {
@@ -606,8 +608,8 @@ func (a *App) getPluginsFromFolder() (map[string]*pluginSignaturePath, *model.Ap
 		}
 	}
 	for _, path := range fileStorePaths {
-		if strings.HasSuffix(path, ".sig") {
-			id := strings.TrimSuffix(filepath.Base(path), ".sig")
+		if strings.HasSuffix(path, ".tar.gz.sig") {
+			id := strings.TrimSuffix(filepath.Base(path), ".tar.gz.sig")
 			if val, ok := pluginSignaturePathMap[id]; !ok {
 				mlog.Error("Unknown signature", mlog.String("path", path))
 			} else {
@@ -615,5 +617,6 @@ func (a *App) getPluginsFromFolder() (map[string]*pluginSignaturePath, *model.Ap
 			}
 		}
 	}
+
 	return pluginSignaturePathMap, nil
 }