Snap lists implicitly installed packages, which causes `metapac clean` to fail.

[VinceGuidry]

Hi, when I use metapac to manage snap packages, the snaps install dependencies which then show up as unmanaged, requiring manual dependency management.

vince@vince-framework13:~/src/shared$ metapac c
[Snap]
bare
core18
core20
gnome-3-28-1804
gtk-common-themes
snapd


these packages will be removed

do you want to continue? yes
[sudo] password for vince:
error: cannot remove "bare", "core18", "core20", "gnome-3-28-1804", "gtk-common-themes", "snapd":
       snap "bare" is not removable: snap is being used by snap gtk-common-themes.
Error:
   0: command failed: "sudo snap remove bare core18 core20 gnome-3-28-1804 gtk-common-themes snapd"

Location:
   src/cmd.rs:82

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.

[ripytide]

I think this is due to our current lack of a method to check which installed snaps are explicitly vs implicitly installed. Unfortunately, like many of our backends there is no simple flag or alternative command available from the snap program which allows us to filter out implicit packages so we are left with alternative methods.

The stack exchange posts I've found don't work on my machine (arch linux), which I suspect is because snaps are designed primarily for ubuntu machines only. I'm not really prepared to install a ubuntu machine in a VM to test things myself.

Links:
https://askubuntu.com/questions/1267967/how-can-i-determine-which-snaps-are-dependencies-and-which-are-manually-install
https://askubuntu.com/questions/1236140/how-to-identify-snaps-on-my-system-i-no-longer-need
https://askubuntu.com/questions/1349738/how-to-list-the-installed-snaps-on-ubuntu/1349743#1349743
https://askubuntu.com/questions/1261242/how-to-list-packages-manually-installed-by-snap

As things currently stand I'm going to leave this issue open until someone submits a PR (probably someone running an ubuntu based distro) that can list only explicitly installed snap packages.