Fix build-in plugins failing to load in windows (#25982)

pkg/plugins/frontend_plugin.go

@@ -3,6 +3,7 @@ package plugins
 import (
 	"net/url"
 	"path"
+	"path/filepath"
 	"strings"
 
 	"github.com/grafana/grafana/pkg/setting"
@@ -63,7 +64,9 @@ func (fp *FrontendPluginBase) handleModuleDefaults() {
 	// Previously there was an assumption that the plugin directory
 	// should be public/app/plugins/<plugin type>/<plugin id>
 	// However this can be an issue if the plugin directory should be renamed to something else
-	currentDir := path.Base(fp.PluginDir)
+	currentDir := filepath.Base(fp.PluginDir)
+	// use path package for the following statements
+	// because these are not file paths
 	fp.Module = path.Join("app/plugins", fp.Type, currentDir, "module")
 	fp.BaseUrl = path.Join("public/app/plugins", fp.Type, currentDir)
 }