avahi: move package to by-name and add avahi-compat override

- Move avahi package from pkgs/development/libraries to pkgs/by-name/av/avahi
  (rename path for consistency with by-name layout).
- Add avahi-compat overlay at pkgs/by-name/av/avahi-compat that enables
  withLibdnssdCompat via avahi.override.
- Remove the old avahi and avahi-compat entries from top-level all-packages.nix.

This keeps the package index organized under by-name and provides a
convenient avahi-compat alias for consumers that need libdnssd compatibility.

pkgs/by-name/av/avahi-compat/package.nix

@@ -0,0 +1,11 @@
+{
+  avahi,
+  ...
+}@args:
+
+avahi.override (
+  {
+    withLibdnssdCompat = true;
+  }
+  // removeAttrs args [ "avahi" ]
+)

pkgs/top-level/all-packages.nix

@@ -1565,10 +1565,6 @@ with pkgs;
 
   apprise = with python3Packages; toPythonApplication apprise;
 
-  avahi-compat = callPackage ../by-name/av/avahi/package.nix {
-    withLibdnssdCompat = true;
-  };
-
   babelfish = callPackage ../shells/fish/babelfish.nix { };
 
   bat-extras = recurseIntoAttrs (lib.makeScope newScope (import ../tools/misc/bat-extras));