Prepare support for other backends

[meisenzahl]

This serves as preparation to be able to support other installer backends.

[meisenzahl]

I successfully tested this with elementaryos-8.0-daily.20240925.iso.

Steps to test this

• Try Demo Mode
• Add # to deb cdrom in /etc/apt/sources.list
• sudo apt update
• sudo apt install git
• git clone https://github.com/elementary/installer
• cd installer
• git checkout prepare-backends
• sudo apt build-dep io.elementary.installer
• meson build --prefix=/usr
• ninja -C build
• sudo ninja -C build install
• sudo systemctl daemon-reload
• sudo systemctl restart io.elementary.InstallerDaemon
• Log Out
• Install as usual

[danirabbit]

A couple questions :)

[danirabbit]

@meisenzahl Would it be too much to ask to propose the hostname validator bit as a separate branch? I think that's probably worth its own commit

[danirabbit]

This looks good to me! I can confirm that the new DistinstBackend file is basically what was in Daemon plus a few methods to convert from Distinst structs to our internal structs