bricks/_common/micropython: Run first mpy instead of __main__

Default to running the first `mpy` file instead of searching for __main__ in
the metadata. Now hosts can specify the actual file name so we can include
it in on-brick UIs like on EV3.

This is distinct from __name__ = "__main__" which will still work, just as it
does in normal Python.

See pybricks/support#2364

Author: laurensvalk

bricks/_common/micropython.c

@@ -234,11 +234,8 @@ static void run_user_program(void) {
     if (nlr_push(&nlr) == 0) {
         nlr_set_abort(&nlr);
 
-        mpy_info_t *info = mpy_data_find(MP_QSTR___main__);
-
-        if (!info) {
-            mp_raise_msg(&mp_type_RuntimeError, MP_ERROR_TEXT("no __main__ module"));
-        }
+        // Main program is in the first mpy file.
+        mpy_info_t *info = mpy_first;
 
         // This is similar to __import__ except we don't push/pop globals
         mp_reader_t reader;