PA: Streamline policy file loading

This is a follow-up to https://github.com/letsencrypt/boulder/issues/8700

Creation of a PA comes in two steps: first policy.New(), which never returns an error, followed by a call to pa.LoadIdentPolicyFile(). As a result, the AuthorityImpl has to have a mutex protecting its internal maps from concurrent writes, and many of its methods have to have guards checking that the member fields they care about aren't nil. But in practice, we only ever call LoadIdentPolicyFile once: from main.go, during service startup. There's no reason for it to be a separate function.

Instead, let's modify policy.New() to take the policy files as an argument and initialize its internal maps immediately. This will remove the need for the mutex and the top-of-method guards.

Start in main.go and trace policy.New(), LoadIdentPolicyFile(), and AuthorityImpl to find all construction and loading call sites. Update the initialization flow so policy.New() receives the policy files and initializes its maps immediately; done means the one-time loader, mutex, and nil guards are no longer needed without changing startup behavior.