installation - installing apk in /system/app with android 5.1 or higher (simple adb push doesn't work anynore)

maybe this is a newbie question, but I'm struggling to find an answer...

I need to install some apk as system apps. In past (up to android 5.01) I was used to just adb push to /system/app and everything was working fine.

I found out today that since Android 5.1 this doesn't work anymore as apps in that directory are not anymore simple apks but they are folders with an arm directory with actual apk and other files.

How can I now install as system apps the apks that I have?

I have sources only for 1 of them.

thanks in advance for your support

regards, blackdir