CI: add check for unused files in pmaports

Add a job, that goes through each dir in pmaports that has an APKBUILD file. For each file in that directory, check if it is mentioned in the APKBUILD source variable, and if it is not, complain about it and fail the check.

Make sure that this works with subdirectories too, like in device-samsung-golden (all files must be mentioned in source, any extra files in subdirectories are also errors).

device-samsung-golden
├── APKBUILD
├── deviceinfo
├── downstream
│   ├── init-usb-hook.sh
│   ├── module-config.conf
│   └── modules-load.conf
└── kwin.sh