Split GNOME Software deb support to separate package etc

This patch set splits GNOME Software's .deb support to a separate package. This has been asked by several people particularly on the Ubuntu side but also by @agx in https://bugs.debian.org/1066030 . Ubuntu's Snap Store supports Snaps and .debs but it can't handle Flatpaks so some people want a Flatpak-only app. There are other combinations that people want.

I've also dropped the patch that hide's upstream's Software Sources page. If the .deb plugin package is installed, it shows .deb sources option that are not very appropriate for how Debian lays out /etc/apt/sources.list (and it can't read the new deb822 style /etc/apt/sources.list.d/ubuntu.sources that Ubuntu is using and perhaps Debian would switch to because it's pretty nice). It also shows the fwupd sources since I left that plugin in the core gnome-software package.

As future easy work, perhaps an additional button to launch software-properties-gtk would be helpful, but directly hiding upstream's sources dialogs wasn't good for Flatpak users.

On the Ubuntu side, I am updating the package to no longer recommend the Snap plugin. It's not needed for several workflows and the recommends could just be in the way. Also I've removed the fwupd plugin since Ubuntu Desktop includes a Snap app to handle firmware updates and several Ubuntu desktop flavors either include it or are not that interested in firmware updates. (Of course there is also gnome-firmware as a standalone app.)

cc/ @mak @agx @smcv for review of the split (you can ignore the Ubuntu part)

I'd like to land this soon since I'm late enough that I will need to request an Ubuntu freeze exception, so maybe you could take a look in the next few days if you're interested.