chore(tvix/nar-bridge): move to Export from storev1pb

This removes the Export method in nar-bridge, and updates all users to
the version now in storev1pb.

It moves the roundtrip test to the importer crate, and some of the
utility functions into a separate util_test.go file.

Change-Id: I81d9e0b35dfd78ef1042bed307281eecd2aaa2a8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9603
Reviewed-by: Brian McGee <brian@bmcgee.ie>
Tested-by: BuildkiteCI

tvix/nar-bridge/pkg/http/nar_get.go

@@ -13,7 +13,6 @@ import (
 	"sync"
 
 	castorev1pb "code.tvl.fyi/tvix/castore/protos"
-	"code.tvl.fyi/tvix/nar-bridge/pkg/exporter"
 	storev1pb "code.tvl.fyi/tvix/store/protos"
 	"github.com/go-chi/chi/v5"
 	nixhash "github.com/nix-community/go-nix/pkg/hash"
@@ -94,7 +93,7 @@ func renderNar(
 	}
 
 	// render the NAR file
-	err := exporter.Export(
+	err := storev1pb.Export(
 		w,
 		pathInfo,
 		func(directoryDigest []byte) (*castorev1pb.Directory, error) {