refactor(3p/overlays): Fetch emacs-overlay through niv

Change-Id: I1c59d463eb1f50fa6e24774ec75ccd2e2d98e120
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5144
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/overlays/emacs.nix

@@ -1,12 +1,4 @@
 # Emacs overlay from https://github.com/nix-community/emacs-overlay
-{ ... }:
+{ depot, ... }:
 
-let
+import depot.third_party.sources.emacs-overlay
-  # from 2022-01-04
-  commit = "a463c3bcbb04b4b744a259587081786ded8fd5b5";
-  src = builtins.fetchTarball {
-    url = "https://github.com/nix-community/emacs-overlay/archive/${commit}.tar.gz";
-    sha256 = "1b7rmshf1wc9wcml7jlzggdzilj644brk5m49fry6lv53vqmykjq";
-  };
-in
-import src

third_party/sources/sources.json

@@ -1,4 +1,16 @@
 {
+    "emacs-overlay": {
+        "branch": "master",
+        "description": "Bleeding edge emacs overlay [maintainer=@adisbladis] ",
+        "homepage": "",
+        "owner": "nix-community",
+        "repo": "emacs-overlay",
+        "rev": "d43cfd1815d2dbae970705196e2a297c2ba87811",
+        "sha256": "1sr7dyq34dfhxhxrp0zjlcf2bpqj1ns5ikhx6d22c5r3zcg507vd",
+        "type": "tarball",
+        "url": "https://github.com/nix-community/emacs-overlay/archive/d43cfd1815d2dbae970705196e2a297c2ba87811.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "nixpkgs": {
         "branch": "nixos-unstable",
         "description": "Nix Packages collection",