Export will traverse a given PathInfo structure, and write the contents
in NAR format to the passed Writer.
It uses directoryLookupFn and blobLookupFn to resolve references.
This is being moved over from nar-bridge. We need to keep the code there
around until we can bump go.mod to storev1 with this merged, but the
tests can already be moved entirely.
Change-Id: Ie0de3077b09344cafa00ff1e2ddb8b52e9e631bc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9602
Tested-by: BuildkiteCI
Reviewed-by: Brian McGee <brian@bmcgee.ie>
Autosubmit: flokli <flokli@flokli.de>
