feat(tools/magrathea): bootstrap a tool for working with depot

this is going to be a serious version of //tools/depot-build.

right now it doesn't support parsing any target specs yet, so only
shells and builds for the physical project of the current folder work.

Change-Id: I4308e29da940571622ff9e539fbb8ededd27aca7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4335
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: grfn <grfn@gws.fyi>

tools/magrathea/default.nix

@@ -0,0 +1,23 @@
+# magrathea helps you build planets
+#
+# it is a tool for working with monorepos in the style of tvl's depot
+{ pkgs, ... }:
+
+pkgs.stdenv.mkDerivation {
+  name = "magrathea";
+  src = ./.;
+  dontInstall = true;
+
+  nativeBuildInputs = [ pkgs.chicken ];
+  buildInputs = with pkgs.chickenPackages.chickenEggs; [
+    matchable
+    srfi-13
+  ];
+
+  propagatedBuildInputs = [ pkgs.git ];
+
+  buildPhase = ''
+    mkdir -p $out/bin
+    csc -o $out/bin/mg -static ${./mg.scm}
+  '';
+}