py: Automatically provide weak links from "foo" to "ufoo" module name.

This commit implements automatic module weak links for all built-in
modules, by searching for "ufoo" in the built-in module list if "foo"
cannot be found.  This means that all modules named "ufoo" are always
available as "foo".  Also, a port can no longer add any other weak links,
which makes strict the definition of a weak link.

It saves some code size (about 100-200 bytes) on ports that previously had
lots of weak links.

Some changes from the previous behaviour:
- It doesn't intern the non-u module names (eg "foo" is not interned),
  which saves code size, but will mean that "import foo" creates a new qstr
  (namely "foo") in RAM (unless the importing module is frozen).
- help('modules') no longer lists non-u module names, only the u-variants;
  this reduces duplication in the help listing.

Weak links are effectively the same as having a set of symbolic links on
the filesystem that is searched last.  So an "import foo" will search
built-in modules first, then all paths in sys.path, then weak links last,
importing "ufoo" if it exists.  Thus a file called "foo.py" somewhere in
sys.path will still have precedence over the weak link of "foo" to "ufoo".

See issues: #1740, #4449, #5229, #5241.

py/objmodule.c

@@ -3,7 +3,7 @@
  *
  * The MIT License (MIT)
  *
- * Copyright (c) 2013, 2014 Damien P. George
+ * Copyright (c) 2013-2019 Damien P. George
  * Copyright (c) 2014-2015 Paul Sokolovsky
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
@@ -26,6 +26,7 @@
  */
 
 #include <stdlib.h>
+#include <string.h>
 #include <assert.h>
 
 #include "py/objmodule.h"
@@ -235,14 +236,6 @@ STATIC const mp_rom_map_elem_t mp_builtin_module_table[] = {
 
 MP_DEFINE_CONST_MAP(mp_builtin_module_map, mp_builtin_module_table);
 
-#if MICROPY_MODULE_WEAK_LINKS
-STATIC const mp_rom_map_elem_t mp_builtin_module_weak_links_table[] = {
-    MICROPY_PORT_BUILTIN_MODULE_WEAK_LINKS
-};
-
-MP_DEFINE_CONST_MAP(mp_builtin_module_weak_links_map, mp_builtin_module_weak_links_table);
-#endif
-
 // returns MP_OBJ_NULL if not found
 mp_obj_t mp_module_get(qstr module_name) {
     mp_map_t *mp_loaded_modules_map = &MP_STATE_VM(mp_loaded_modules_dict).map;
@@ -267,6 +260,21 @@ void mp_module_register(qstr qst, mp_obj_t module) {
     mp_map_lookup(mp_loaded_modules_map, MP_OBJ_NEW_QSTR(qst), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = module;
 }
 
+#if MICROPY_MODULE_WEAK_LINKS
+// Search for u"foo" in built-in modules, return MP_OBJ_NULL if not found
+mp_obj_t mp_module_search_umodule(const char *module_str) {
+    for (size_t i = 0; i < MP_ARRAY_SIZE(mp_builtin_module_table); ++i) {
+        const mp_map_elem_t *entry = (const mp_map_elem_t*)&mp_builtin_module_table[i];
+        const char *key = qstr_str(MP_OBJ_QSTR_VALUE(entry->key));
+        if (key[0] == 'u' && strcmp(&key[1], module_str) == 0) {
+            return (mp_obj_t)entry->value;
+        }
+
+    }
+    return MP_OBJ_NULL;
+}
+#endif
+
 #if MICROPY_MODULE_BUILTIN_INIT
 void mp_module_call_init(qstr module_name, mp_obj_t module_obj) {
     // Look for __init__ and call it if it exists