ArcLoader requires locale directory to exist to even use fallbacks

Using the ArcLoader is really picky and I'm having a hard time setting it up for use (adding Fluent functions to a tera CLI). The shared resources feature works, as do fallback locales. The trouble is neither work if a directory in the locales path does not exist for every possible locale variant that might be used. Even if the message data would otherwise fall back to another locale or the shared resources, an empty directory needs to exist to stuff the bundles.

c.f. #41