avoid exporting non-annexed symlinks

avoid exporting non-annexed symlinks

So that importing does not replace them with plain files.

This works similarly to how the previous handling of submodules and
matchers did, except that annexed symlinks still get exported as plain
files of course, it's only non-annexed symlinks that it does not make sense
to export.

When symlinks have previously been exported, updating the export will
unexport them after upgrading to this commit.

Sponsored-by: Kevin Mueller on Patreon