py/builtinimport: Optimise sub-package loading.

This makes it so that sub-packages are resolved relative to their parent's
`__path__`, rather than re-resolving each parent's filesystem path.

The previous behavior was that `import foo.bar` would first re-search
`sys.path` for `foo`, then use the resulting path to find `bar`.

For already-loaded and u-prefixed modules, because we no longer need to
build the path from level to level, we no longer unnecessarily search
the filesystem. This should improve startup time.

Explicitly makes the resolving process clear:
 - Loaded modules are returned immediately without touching the filesystem.
 - Exact-match of builtins are also returned immediately.
 - Then the filesystem search happens.
 - If that fails, then the weak-link handling is applied.

This maintains the existing behavior: if a user writes `import time` they
will get time.py if it exits, otherwise the built-in utime. Whereas `import
utime` will always return the built-in.

This also fixes a regression from a7fa18c203
where we search the filesystem for built-ins. It is now only possible to
override u-prefixed builtins. This will remove a lot of filesystem stats
at startup, as micropython-specific modules (e.g. `pyb`) will no longer
attempt to look at the filesystem.

Added several improvements to the comments and some minor renaming and
refactoring to make it clearer how the import mechanism works. Overall
code size diff is +56 bytes on STM32.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/obj.h

@@ -1201,8 +1201,6 @@ typedef struct _mp_obj_module_t {
 static inline mp_obj_dict_t *mp_obj_module_get_globals(mp_obj_t module) {
     return ((mp_obj_module_t *)MP_OBJ_TO_PTR(module))->globals;
 }
-// check if given module object is a package
-bool mp_obj_is_package(mp_obj_t module);
 
 // staticmethod and classmethod types; defined here so we can make const versions
 // this structure is used for instances of both staticmethod and classmethod