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.

ports/cc3200/mpconfigport.h

@@ -169,19 +169,6 @@ extern const struct _mp_obj_module_t mp_module_ussl;
     { MP_ROM_QSTR(MP_QSTR_ubinascii),   MP_ROM_PTR(&mp_module_ubinascii) }, \
     { MP_ROM_QSTR(MP_QSTR_ussl),        MP_ROM_PTR(&mp_module_ussl) },      \
 
-#define MICROPY_PORT_BUILTIN_MODULE_WEAK_LINKS \
-    { MP_ROM_QSTR(MP_QSTR_errno),       MP_ROM_PTR(&mp_module_uerrno) },    \
-    { MP_ROM_QSTR(MP_QSTR_struct),      MP_ROM_PTR(&mp_module_ustruct) },   \
-    { MP_ROM_QSTR(MP_QSTR_re),          MP_ROM_PTR(&mp_module_ure) },       \
-    { MP_ROM_QSTR(MP_QSTR_json),        MP_ROM_PTR(&mp_module_ujson) },     \
-    { MP_ROM_QSTR(MP_QSTR_os),          MP_ROM_PTR(&mp_module_uos) },       \
-    { MP_ROM_QSTR(MP_QSTR_time),        MP_ROM_PTR(&mp_module_utime) },     \
-    { MP_ROM_QSTR(MP_QSTR_select),      MP_ROM_PTR(&mp_module_uselect) },   \
-    { MP_ROM_QSTR(MP_QSTR_socket),      MP_ROM_PTR(&mp_module_usocket) },   \
-    { MP_ROM_QSTR(MP_QSTR_binascii),    MP_ROM_PTR(&mp_module_ubinascii) }, \
-    { MP_ROM_QSTR(MP_QSTR_ssl),         MP_ROM_PTR(&mp_module_ussl) },      \
-    { MP_ROM_QSTR(MP_QSTR_machine),     MP_ROM_PTR(&machine_module) },      \
-
 // extra constants
 #define MICROPY_PORT_CONSTANTS \
     { MP_ROM_QSTR(MP_QSTR_umachine),     MP_ROM_PTR(&machine_module) },      \