Expose the same ReadJSONFile function to simulation, GUI and rmgen.

This way globalscripts/ can use the same code in all contexts, all paths are absolute and only one function name is needed.
Remove ReadCivJSONFile which is redundant with this function as well.
Restrict read access of rmgen/ to rmgen/ and simulation/, refs rP20507, rP20528.

Refs #4868
Differential Revision: https://code.wildfiregames.com/D1085
Reviewed By: wraitii
Partial agreement with leper