The current version of the content package mane plugin there is not support for an installation of a package into Sling / Composum.
We at headwire.com forked the project and started to work on a basic version were we can just install a package (bundleStatusURL=-).
The plugin fails in the PackageInstaller.installFile() where the URL is appended with a CRX specific string "/.json?cmd=upload ". In addition the response is required to be JSon in a particular format.
Some minor changes to PageInstall, PackageManagerHelper and a new PackageManagerHtmlCall class does make the plain installation work.
I suggest to create a Factory that can provide an Package Client class based on the package url and that Package Client is then able to handle Composum, CRX or whatever comes down the road.