You should _never_ use `$base_url` explicitly... Additionally, `url()` is inappropriate for this use, as it will try to add in language prefixes when multiple languages are in use.
Use `file_create_url()`, so:
```php
$schema = file_create_url("$critical_edition_module_path/CWRC-Writer/src/schema/CWRC-TEIBasic.rng");
```