This commit moves all of the PHP code from `plugins/woocommerce-admin`
to its new home in `plugins/woocommerce`.
- `plugins/woocommerce-admin/src` -> `plugins/woocommerce/src/Admin`
- `plugins/woocommerce-admin/src-internal` -> `plugins/woocommerce/src/Internal`
- `plugins/woocommerce-admin/includes` -> `plugins/woocommerce/includes/react-admin`