refactor(tools/emacs): Move third-party overrides to //third_party

EXWM and vterm are overridden, but this shouldn't be happening outside
of //third_party

third_party/emacs/exwm.nix

@@ -0,0 +1,13 @@
+# EXWM straight from GitHub. As of 2020-05-15, XELB in nixpkgs is
+# already at a recent enough version and does not need to be
+# overridden.
+{ pkgs, ... }:
+
+pkgs.emacsPackages.exwm.overrideAttrs(_: {
+  src = pkgs.fetchFromGitHub {
+    owner = "ch11ng";
+    repo = "exwm";
+    rev = "48db94f48bea1137132345abfe8256cfc6219248";
+    sha256 = "0jj12z6m5kvanq19gds3jpvid2mg8w28bbbq9iycl751y2sj4l1r";
+  };
+})

third_party/emacs/vterm.nix

@@ -0,0 +1,11 @@
+# Overridden vterm to fetch a newer version
+{ pkgs, ... }:
+
+pkgs.emacsPackages.vterm.overrideAttrs(_: {
+  src = pkgs.fetchFromGitHub{
+    owner = "akermu";
+    repo = "emacs-libvterm";
+    rev = "58b4cc40ee9872a08fc5cbfee78ad0e195a3306c";
+    sha256 = "1w5yfl8nq4k7xyldf0ivzv36vhz3dwdzk6q2vs3xwpx6ljy52px6";
+  };
+})