Refactor BuilderCAI static caches into new BuilderCaches file.

[saurtron]

Work done

• Factor out static caches from BuilderCAI

Remarks

• Just moving code around.
• Part of a bigger effort to allow builder commands to be used by other unit and ai types.
  • See Allow buildings to assist construction #1597 (comment)
  • See saurtron@b3acf9a#diff-f07ea03231eeb0aab27b40904941d7c552446b68667f4fca981c77768e59e4a6R421
• static functions may need to be used in other modules in the future
  • even if that's not the case also cleaner this way
• This can probably be refactored even more internally to be more flexible in the future
  • Will likely need to become a CEventClient to be able to address TODOs there.

[sprunk]

If it's an instanceless class (not even a singleton needed) perhaps it should be a namespace with regular functions? See #1651 (comment)

[sprunk]

Previously it was obvious what "members" were because this chunk was right under a bunch of creg registrations

Suggested change

	// not adding to members, should repopulate itself
	// not adding to creg, should repopulate itself

[sprunk]

Probably needs the licence notice at the top (as Spring, since it's entirely old code)

Suggested change

	#include "Sim/Units/CommandAI/BuilderCaches.h"
	/* This file is part of the Spring engine (GPL v2 or later), see LICENSE.html */
	#include "Sim/Units/CommandAI/BuilderCaches.h"