Move duplicate token check to token_requirements()

To allow #881270: [bookpath], [menupath], [*path] tokens not cleaned: aliases without punctuation removed, lower casing, etc. to proceed, we need to roll back #587148: Handle tokens correctly when more than one module defines a given token because token.module should *not* alter the token values itself. Rather we should add token_requirements and check for duplicate token definitions using hook_token_list(). That way, we can actually show which modules are the ones that conflict, rather than showing a watchdog message every time tokens are used.