refactor: Replace 'depotPath' with 'depot.path'

Instead of having two ways of accessing the path to the depot (one of
which was stuttering, depot.depotPath) we settle on only one:
depot.path.

This was mostly used for NixOS module imports.

Co-Authored-By: Florian Klink <flokli@flokli.de>
Change-Id: I2c0db23383fc34f6ca76baaad4cc4af2d9dfae15
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2962
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>

ops/modules/www/wigglydonke.rs.nix

@@ -9,7 +9,7 @@
     services.nginx.virtualHosts."wigglydonke.rs" = {
       enableACME = true;
       forceSSL = true;
-      root = "${depot.depotPath}/users/grfn/wigglydonke.rs";
+      root = "${depot.path}/users/grfn/wigglydonke.rs";
     };
   };
 }