Add ".wren" to modules in the embedder.

test/module/shared_import/a.wren

@@ -1,5 +1,5 @@
 // nontest
 IO.print("a")
-var Shared = "shared.wren".import_("Shared")
+var Shared = "shared".import_("Shared")
 var A = "a " + Shared
 IO.print("a done")

test/module/shared_import/b.wren

@@ -1,5 +1,5 @@
 // nontest
 IO.print("b")
-var Shared = "shared.wren".import_("Shared")
+var Shared = "shared".import_("Shared")
 var B = "b " + Shared
 IO.print("b done")

test/module/shared_import/shared_import.wren

@@ -1,5 +1,5 @@
-var A = "a.wren".import_("A")
-var B = "b.wren".import_("B")
+var A = "a".import_("A")
+var B = "b".import_("B")
 
 // Shared module should only run once:
 // expect: a