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>

web/bubblegum/default.nix

@@ -207,7 +207,7 @@ let
       ] ++ [
         "${bins.nint}"
         # always pass depot so scripts can use this library
-        "--arg depot '(import ${depot.depotPath} {})'"
+        "--arg depot '(import ${depot.path} {})'"
       ]);
     in runExecline.local drvName {} [
       "importas" "out" "out"