loader: Move schema loading into loader package

This commit moves the schema loading into the loader package so that
it can be reused. Also, the schema loading implementation has been
refactored a bit:

* Errors are more consistent with other loader errors
* Reduced a small amount of duplication on file reading
* Replaced a few nested blocks with early exits

Signed-off-by: Torin Sandall <torinsandall@gmail.com>