Change how customFields are built

Instead of passing an expression (which cannot come from the current
module), we pass a name, which is allowed to reference functions from
the module we're in. Since we currently don't have custom fields, we
don't need to modify any callers.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>