Use packageindex.HasValidExtension() in package index detection code

I made a dedicated function for this purpose, then forgot to use it!

Author: per1234

project/project.go

@@ -266,7 +266,7 @@ func isPackageIndex(potentialProjectPath *paths.Path) bool {
 }
 
 func isPackageIndexIndicatorFile(filePath *paths.Path) bool {
-	if filePath.Ext() == ".json" {
+	if packageindex.HasValidExtension(filePath) {
 		return true
 	}