emacs: Add custom elpa fetcher

Elpa only serves the latest version of a given package uncompressed.
Once that release is no longer the latest & greatest it gets archived and compressed
meaning that both the URL and the hash changes.

To work around this issue we fall back to the URL with the .lz suffix and if that's the
one we downloaded we uncompress the file to ensure the hash matches regardless of compression.

pkgs/applications/editors/emacs/elisp-packages/fetchelpa.nix

@@ -0,0 +1,21 @@
+# Elpa only serves the latest version of a given package uncompressed.
+# Once that release is no longer the latest & greatest it gets archived and compressed
+# meaning that both the URL and the hash changes.
+#
+# To work around this issue we fall back to the URL with the .lz suffix and if that's the
+# one we downloaded we uncompress the file to ensure the hash matches regardless of compression.
+
+{ fetchurl, lzip }:
+
+{ url, ... }@args: fetchurl ((removeAttrs args [ "url" ]) // {
+  urls = [
+    url
+    (url + ".lz")
+  ];
+  postFetch = ''
+    if [[ $url == *.lz ]]; then
+      ${lzip}/bin/lzip -c -d $out > uncompressed
+      mv uncompressed $out
+    fi
+  '';
+})