Make resolveResources() and packageResources() private on CelerityResourceMap

Summary: Ref T4222. These are the last two "return a big ball of mud" methods. Make the API stronger so I can swap out the implementations.

Test Plan: Reloaded pages.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T4222

Differential Revision: https://secure.phabricator.com/D7871