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>

third_party/nix/default.nix

@@ -3,7 +3,6 @@ args@{
 , pkgs ? depot.third_party.nixpkgs
 , lib
 , buildType ? "release"
-, depotPath ? ../..
 , ...
 }:
 
@@ -198,7 +197,7 @@ in lib.fix (self: pkgs.llvmPackages.libcxxStdenv.mkDerivation {
       installCheckInputs = up.installCheckInputs ++ [run_clang_tidy];
 
       shellHook = ''
-        export NIX_DATA_DIR="${toString depotPath}/third_party"
+        export NIX_DATA_DIR="${toString depot.path}/third_party"
         export NIX_TEST_VAR=foo
       '';
     });