forgejo: rename package.nix to generic.nix in preparation for multiple versions

Otherwise, git wouldn't be able to detect and flag this as a rename and
as such would lose us valuable history when using git blame or git log.

pkgs/top-level/all-packages.nix

@@ -2193,6 +2193,8 @@ with pkgs;
 
   diff-so-fancy = callPackage ../applications/version-management/diff-so-fancy { };
 
+  forgejo = callPackage ../by-name/fo/forgejo/generic.nix { };
+
   gex = callPackage ../applications/version-management/gex {
     inherit (darwin.apple_sdk.frameworks) Security;
   };