feat: Add 'depotPath' to depot root import

This is useful for things like including NixOS modules in
configurations without creating long and error-prone relative paths.

Change-Id: I4a5ebb1a0e5adf90b6bc50e884db453e12461001
Reviewed-on: https://cl.tvl.fyi/c/depot/+/243
Reviewed-by: lukegb <lukegb@tvl.fyi>

default.nix

@@ -54,6 +54,10 @@ in fix(self: {
     __functor = x: (readTree' x.config);
     config = self.config;
   };
+
+  # Make the path to the depot available for things that might need it
+  # (e.g. NixOS module inclusions)
+  depotPath = ./.;
 }
 
 # Add local packages as structured by readTree