Any packages installed in binary hooks are marked as manually installed - even dependencies

After much debugging trying to understand why `apt-get autoremove` was not working in a binary hook I realised that all packages installed in a binary hook are marked as manually installed - even the dependencies of packages. This is a bug.

Another side affect of this is that any dependency of a package in the base chroot that is removed in a binary hook is not autoremovable in the binary hook.

It is autoremovable once the image is created and launched though.

This is due to how the `setup_mountpoint` function works (see https://git.launchpad.net/livecd-rootfs/tree/live-build/functions#n123) specifically

```
mount -t tmpfs none "$mountpoint/var/lib/apt"
```

Which means that inside the chroot that the binary hook works on, the `/var/lib/apt` directory is empty... including the `/var/lib/apt/extended_states` file which is used to store whether a package was auto installed or not.

The same goes the other way - any changes to the `/var/lib/apt/extended_states` file inside chroot that the binary hook is changing are not reflected in the final image due to the `teardown_mountpoint` (see https://git.launchpad.net/livecd-rootfs/tree/live-build/functions#n152) not persisting any of those changes.

The end result being.

* If a package is removed in the binary hook then the resulting image will likely have autoremovable packages at first launch
* Any package and their dependencies installed in a binary hook will all be marked as manually installed.

SRU
====

[ Impact ]

 * Packages installed in a binary hook are marked as manually installed - even the dependencies of packages

[ Test Plan ]

 * Build test images and remove a package with dependencies.
   * Confirm that the dependencies are marked as autoremovable.
   * Confirm there are packages marked as auto installed

[ Where problems could occur ]

 * Is any dependent packages are not marked as autremovable.
 * If no packages are marked as auto installed