core: Do create hardlinks to symlinks for checkouts

core: Do create hardlinks to symlinks for checkouts

I was noticing a recent performance issue with checkouts
which seemed to be mostly us going back to doing a `fsync()` on
directories.

Regardless, while looking at that, I saw we were spending time
creating new symlinks. Even though symlinks are small, it's still
better to hardlink them.

Going way back in time, the reason we weren't doing this is
because we were hitting `EMFILE` on ext4, but that was for
gnome-continuous which creates *many* build roots.  Even
there though, they're just a cache, and we handle `EMFILE`.

For ostree-for-host-system, we don't expect to have many roots (just 3
at most transiently), so hardlinking symlinks does make sense.

Closes: #521
Approved by: jlebon