Convert maintainer file entries to attributes, add github handles

Based on https://github.com/NixOS/nixpkgs/pull/34842, the
nix-instantiate output was pretty-printed and the validity of the github handles
manually verified, by automatically checking whether the user handles exist on
github (https://github.com/userhandle, status 200 or 404).
Each handle under 5 characters was manually checked (because the collision
probability with non-maintainer accounts is high), each missing entry was
manually researched.

The script used is kept in `maintainers/scripts` as an example of how to work
with the mainainers list through nix’ JSON interface.

lib/default.nix

@@ -21,7 +21,7 @@ let
 
     # packaging
     customisation = callLibs ./customisation.nix;
-    maintainers = callLibs ./maintainers.nix;
+    maintainers = import ./maintainers.nix;
     meta = callLibs ./meta.nix;
     sources = callLibs ./sources.nix;
 

maintainers/scripts/check-maintainer-github-handles.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p jq parallel
+
+# Example how to work with the `lib.maintainers` attrset.
+# Can be used to check whether all user handles are still valid.
+
+set -e
+
+# checks whether a user handle can be found on github
+function checkUser {
+    local user="$1"
+    local status=
+    status="$(curl --silent --head "https://github.com/${user}" | grep Status)"
+    printf "%s\t\t\t\t%s\n" "$status" "$user"
+}
+export -f checkUser
+
+# output the maintainers set as json
+# and filter out the github username of each maintainer (if it exists)
+# then check 100 at the same time
+nix-instantiate -A lib.maintainers --eval --strict --json \
+    | jq -r '.[]|.github' \
+    | parallel -j100 checkUser