builds one installable Joomla package per cart, each bundling the payment
method plugin and its settlement scheduler task, so the merchant installs
a single zip.

re-vendors from canonical on every build (engine from xmr-pay-php, the
shared adapter core, and the one payment card) so a package can never ship
stale code, drops an index.html into every directory, and assembles the
pkg_ package from the per-plugin zips. source checkouts default to
siblings of this repo and can be overridden with XMRPAY_ENGINE/XMRPAY_CORE.