The code was sorting files in the archives entirely by path string,
but that's not what fs.WalkDir would do. In a directory with
subdirectory foo/bar and file foo/bar.go, foo/bar gets visited
first, so foo/bar/baz appears before foo/bar.go, even though
"foo/bar/baz" > "foo/bar.go".
This CL replaces the string comparison with a path-aware
comparison that places foo/bar/baz before foo/bar.go,
so that if the tar file is extracted and then repacked using
fs.WalkDir, the files will remain in the same order.
This will make it easier to compare the pristine distpack-produced
tgz for darwin against the rebuilt tgz with signed binaries.