opkg-cl: stop preloading package feeds on install

Instead of unconditionally loading all package feed lists entirely into
memory, let opkg_install_cmd() selectively parse the lists using the new
pkg_hash_load_package_details() helper.

This will result in a slightly longer runtime but dramatically reduces the
memory requirements. On an x86_64 test system, the peak heap memory
consumption during an "opkg install luci" dropped from 3.7 MB to about
360 KB, a decrease of roughly 90%.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>