[v9.5.x] Plugins: Fix module.js file not being closed when loading plugins (#66435)

* Plugins: Fix module.js file not being closed when loading plugins (#66288)

* Plugins: Loader: Fix module.js file not being closed

* Plugins: LocalFS: Add comments, ensure same Read() behaviour as os.File's

* Changed comment for Close()

* Add tests for LocalFS

* "Fix" linter error

* "Fix" linter error again

(cherry picked from commit 1c3ad81826)

* trigger ci

---------

Co-authored-by: Giuseppe Guerra <giuseppe.guerra@grafana.com>
Co-authored-by: Giuseppe Guerra <to@nyo.zz.mu>

pkg/plugins/manager/loader/loader.go

@@ -152,12 +152,16 @@ func (l *Loader) loadPlugins(ctx context.Context, src plugins.PluginSource, foun
 		// verify module.js exists for SystemJS to load.
 		// CDN plugins can be loaded with plugin.json only, so do not warn for those.
 		if !plugin.IsRenderer() && !plugin.IsCorePlugin() {
-			_, err := plugin.FS.Open("module.js")
+			f, err := plugin.FS.Open("module.js")
 			if err != nil {
 				if errors.Is(err, plugins.ErrFileNotExist) && !l.pluginsCDN.PluginSupported(plugin.ID) {
 					l.log.Warn("Plugin missing module.js", "pluginID", plugin.ID,
 						"warning", "Missing module.js, If you loaded this plugin from git, make sure to compile it.")
 				}
+			} else if f != nil {
+				if err := f.Close(); err != nil {
+					l.log.Warn("Could not close module.js", "pluginID", plugin.ID, "err", err)
+				}
 			}
 		}