py/builtinimport: Refactor module importing.

Simplify and document/comment the handling of builtin import for:
- already-loaded modules
- built-in modules
- built-in umodules (formerly weak links)
- filesystem modules

Retains existing functionality with smaller code size but should also
facilitate potential new features (built-in packages, controlling the
frozen path).

Also makes the (unix-only) -m behavior a bit more obvious and configurable.

Code size change with this commit:

   bare-arm:    +0 +0.000%
minimal x86:   -64 -0.039%
   unix x64:   -32 -0.006%
unix nanbox:    -4 -0.001%
      stm32:  -184 -0.047% PYBV10
     cc3200:  -120 -0.065%
    esp8266:  -228 -0.033% GENERIC
      esp32:  -268 -0.018% GENERIC[incl +16(data)]
        nrf:  -152 -0.087% pca10040
        rp2:  -256 -0.052% PICO
       samd:   -80 -0.057% ADAFRUIT_ITSYBITSY_M4_EXPRESS

py/objmodule.h

@@ -30,18 +30,9 @@
 
 extern const mp_map_t mp_builtin_module_map;
 
-mp_obj_t mp_module_get(qstr module_name);
-void mp_module_register(qstr qstr, mp_obj_t module);
-
-mp_obj_t mp_module_search_umodule(const char *module_str);
-
-#if MICROPY_MODULE_BUILTIN_INIT
-void mp_module_call_init(qstr module_name, mp_obj_t module_obj);
-#else
-static inline void mp_module_call_init(qstr module_name, mp_obj_t module_obj) {
-    (void)module_name;
-    (void)module_obj;
-}
+mp_obj_t mp_module_get_loaded_or_builtin(qstr module_name);
+#if MICROPY_MODULE_WEAK_LINKS
+mp_obj_t mp_module_get_builtin(qstr module_name);
 #endif
 
 #endif // MICROPY_INCLUDED_PY_OBJMODULE_H