feat(tvix/store): add import::import_path

This imports the contents at a given Path into the tvix store.

It doesn't register the contents at a Path in the store itself, that's up
to the PathInfoService.

Change-Id: I2c493532d65b90f199ddb7dfc90249f5c2957dee
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8159
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/Cargo.nix

@@ -8370,6 +8370,10 @@ rec {
             packageId = "tracing-subscriber";
             features = [ "json" ];
           }
+          {
+            name = "walkdir";
+            packageId = "walkdir";
+          }
         ];
         buildDependencies = [
           {