print_apt_packages: Go back to apt-cache for non-installed packages

Unfortunately, /var/lib/dpkg/status on recent-ish Debian versions at
least only contains the *installed* packages, rendering this solution
broken.

What we do instead is:

1. Remove a useless newline from each package, so our limit would now
let more full package data sets through
2. Increase the limit by 5x

This yields a completion that runs in ~800ms instead of ~700ms on a
raspberry pi, but gives ~10x the candidates, compared to the old
apt-cache version.

This partially reverts 96deaae7d8

share/functions/__fish_print_apt_packages.fish

@@ -13,25 +13,30 @@ function __fish_print_apt_packages
         return 1
     end
 
-    # Do not not use `apt-cache` as it is sometimes inexplicably slow (by multiple orders of magnitude).
     if not set -q _flag_installed
-        awk '
+        # Do not generate the cache as apparently sometimes this is slow.
-BEGIN {
+        # http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=547550
-  FS=": "
+        # (It is safe to use `sed -r` here as we are guaranteed to be on a GNU platform
-}
+        # if apt-cache was found.)
-
+        # Uses the UTF-8/ASCII record separator (0x1A) character.
-/^Package/ {
+        #
-  pkg=$2
+        # Note: This can include "Description:" fields which we need to include,
-}
+        # "Description-en_GB" (or another locale code) fields which we need to include
-
+        # as well as "Description-md5" fields which we absolutely do *not* want to include
-/^Description(-[a-zA-Z]+)?:/ {
+        # The regex doesn't allow numbers, so unless someone makes a hash algorithm without a number
-  desc=$2
+        # in the name, we're safe. (yes, this should absolutely have a better format).
-  if (index(pkg, "'$search_term'") > 0) {
+        #
-    print pkg "\t" desc
+        # aptitude has options that control the output formatting, but is orders of magnitude slower
-  }
+        #
-  pkg="" # Prevent multiple description translations from being printed
+        # sed could probably do all of the heavy lifting here, but would be even less readable
-}' </var/lib/dpkg/status
+        #
+        # The `head -n 500` causes us to stop once we have 500 lines. We do it after the `sed` because
+        # Debian package descriptions can be extremely long and are hard-wrapped: texlive-latex-extra
+        # has about 2700 lines on Debian 11.
+        apt-cache --no-generate show '.*'(commandline -ct)'.*' 2>/dev/null | sed -r '/^(Package|Description-?[a-zA-Z_]*):/!d;s/Package: (.*)/\1\t/g;s/Description-?[^:]*: (.*)/\1\x1a/g' | head -n 2500 | string join "" | string replace --all --regex \x1a+ \n | uniq
+        return 0
     else
+        # Do not not use `apt-cache` as it is sometimes inexplicably slow (by multiple orders of magnitude).
         awk '
 BEGIN {
   FS=": "