pkg_tar: remap_paths should favor the longest match

Currently, remap_paths will remap each file according to the first match in the map iteration order:
https://github.com/bazelbuild/rules_pkg/blob/e8bff981f600a4faa01d5c8f5dd57d05891c8e2d/pkg/pkg.bzl#L25-L30.

This makes it impossible to have predictable results when you have two keys in remap_paths where one is a prefix of the other. I think we should favor the longest (most specific) match in that case.