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>

bin/__dispatch.sh

@@ -37,6 +37,9 @@ case "${TARGET_TOOL}" in
   meson)
     attr="third_party.nixpkgs.meson"
     ;;
+  mg)
+    attr="tools.magrathea"
+    ;;
   ninja)
     attr="third_party.nixpkgs.ninja"
     ;;

bin/mg

@@ -0,0 +1 @@
+__dispatch.sh