py/frozenmod: Pass the source name of the frozen module to the lexer.

This allows for better error messages, since the name of the file (sans
.py) can now be printed when an exception occurs within a frozen script.

py/frozenmod.c

@@ -50,7 +50,8 @@ STATIC mp_lexer_t *mp_find_frozen_str(const char *str, size_t len) {
     for (int i = 0; *name != 0; i++) {
         size_t l = strlen(name);
         if (l == len && !memcmp(str, name, l)) {
-            mp_lexer_t *lex = MICROPY_MODULE_FROZEN_LEXER(MP_QSTR_, mp_frozen_str_content + offset, mp_frozen_str_sizes[i], 0);
+            qstr source = qstr_from_strn(name, l);
+            mp_lexer_t *lex = MICROPY_MODULE_FROZEN_LEXER(source, mp_frozen_str_content + offset, mp_frozen_str_sizes[i], 0);
             return lex;
         }
         name += l + 1;