fix(nix/fetchGoModule): correctly case-fold mixed-case package paths

Change-Id: I5a93462a23bf3c9f22fc3de0b173763a9bc8d526
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1591
Tested-by: BuildkiteCI
Reviewed-by: edef <edef@edef.eu>

nix/fetchGoModule/default.nix

@@ -1,10 +1,22 @@
-{ pkgs, ... }:
+{ lib, pkgs, ... }:
+
+let
+
+  inherit (lib)
+    lowerChars
+    replaceStrings
+    upperChars
+  ;
+
+  caseFold = replaceStrings upperChars (map (c: "!" + c) lowerChars);
+
+in
 
 { path, version, sha256 }:
 
 (pkgs.fetchurl {
   name = "source";
-  url = "https://proxy.golang.org/${path}/@v/v${version}.zip";
+  url = "https://proxy.golang.org/${caseFold path}/@v/v${version}.zip";
   inherit sha256;
 
   recursiveHash = true;