racket/enter: fix for `enter!' related to submodules

The load handler didn't obey the protocol with respect to a second
argument that is a list starting with #f.

Closes PR 13096

collects/racket/enter.rkt

@@ -64,7 +64,9 @@
         (eprintf "  [~aloading ~a]\n" (if re? "re-" "") path))
       void))
   (lambda (path name)
-    (if name
+    (if (and name
+             (not (and (pair? name)
+                       (not (car name)))))
       ;; Module load:
       (let* ([code (get-module-code
                     path "compiled"
@@ -87,7 +89,7 @@
         ;; Evaluate the module:
         (parameterize ([current-module-declare-source actual-path])
           (eval code)))
-      ;; Not a module:
+      ;; Not a module, or a submodule that we shouldn't load from source:
       (begin (notify path) (orig path name)))))
 
 (define (get-timestamp path)

collects/tests/racket/enter.rkt

@@ -0,0 +1,4 @@
+#lang racket
+(require racket/enter)
+
+(enter! slideshow/pict)