luci-app-opkg: honor installed flag to skip half-installed packages

Do not consider half-installed packages (which happens after an
installation failure) to be installed.

Ref: https://github.com/openwrt/luci/pull/2775
Signed-off-by: Dirk Brenken <dev@brenken.org>
[split into multiple commits, refactored code, use local variables]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>