Add a wrapper around buildCrystalPackage to allow for nonstandard shard configurations.

custom-crystal/default.nix

@@ -0,0 +1,25 @@
+{ stdenv, lib, linkFarm, fetchgit, fetchFromGitHub }:
+
+{ crystal,
+  gitShardsFile ? null,
+  lockFile ? null,
+  shardsFile ? null, ...}@args:
+
+let
+    buildArgs = builtins.removeAttrs args [ "crystal" ];
+    githubLinks = lib.mapAttrsToList (name: value: {
+        inherit name;
+        path = fetchFromGitHub value;
+    }) (import shardsFile);
+    gitLinks = lib.mapAttrsToList (name: value: {
+        inherit name;
+	path = fetchgit { inherit (value) url rev sha256; };
+    }) (import gitShardsFile);
+    crystalLib = linkFarm "crystal-lib" (githubLinks ++ gitLinks);
+    configurePhase = args.configurePhase or lib.concatStringsSep "\n" ([
+        "runHook preConfigure"
+    ] ++ lib.optional (lockFile != null)   "ln -s ${lockFile} ./shard.lock"
+        ++ lib.optional (shardsFile != null) "ln -s ${crystalLib} lib"
+	++ [ "runHook postConfigure "]);
+in
+    crystal.buildCrystalPackage (buildArgs // { inherit configurePhase; })