__fish_print_packages: Extract the rest

Of note: The rpm/yum thing seems to be coupled, so I put it into one
function that tries the yum helper and uses the rpm path otherwise.
Zypper is already its own thing, so this should only be used for yum
and probably dnf (does that still have the helper?)

Zypper can be dropped, as that already used a separate function in the file.

Apk can just be inlined - it's literally one line for installed and another for all packages.

share/functions/__fish_print_apt_packages.fish

@@ -2,18 +2,13 @@ function __fish_print_apt_packages
     argparse --name=__fish_print_packages i/installed -- $argv
     or return
 
-    set -l only_installed 1
-    if not set -q _flag_installed
-        set -e only_installed
-    end
-
     switch (commandline -ct)
         case '-**'
             return
     end
 
     type -q -f apt-cache || return 1
-    if not set -q only_installed
+    if not set -q _flag_installed
         # Do not generate the cache as apparently sometimes this is slow.
         # http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=547550
         # (It is safe to use `sed -r` here as we are guaranteed to be on a GNU platform