middleware/directives: check if package exist on the file system (#767)

directives_generate.go

@@ -43,7 +43,7 @@ func main() {
 		md[priority] = items[1]
 		mi[items[1]] = middlewarePath + items[2] // Default, unless overriden by 3rd arg
 
-		if strings.Contains(items[2], "/") { // External package has been given
+		if _, err := os.Stat(middlewareFSPath + items[2]); err != nil { // External package has been given
 			mi[items[1]] = items[2]
 		}
 	}
@@ -113,7 +113,8 @@ func fatalIfErr(err error) {
 }
 
 const (
-	middlewarePath = "github.com/coredns/coredns/middleware/"
-	middlewareFile = "middleware.cfg"
-	header         = "// generated by directives_generate.go; DO NOT EDIT\n\n"
+	middlewarePath   = "github.com/coredns/coredns/middleware/"
+	middlewareFile   = "middleware.cfg"
+	middlewareFSPath = "middleware/" // Where the middleware packages are located on the file system
+	header           = "// generated by directives_generate.go; DO NOT EDIT\n\n"
 )