Point internal stockpile cut percentages to tradezone specific stockpiles

[sobisonator]

Now the cuts of trade proceeds, including the amount of goods to be subtracted from seller stockpiles, will need to look at tradezone-specific stockpiles rather than one central internal stockpile, as governorships sell into and buy from tradezones rather than a unified internal stockpile.

var:internal_stockpile_cut_percentage_$tradegood$ for example, will need to be more like var:$tradegood$_stockpile_cut_percentage_$tradezone$

[sobisonator]

Closing, now fully functional as of https://github.com/sobisonator/imp19c/commit/7773637a6990b3ee5286ba1301b9cb58aa577855