gwenhywfar: Unify plugin directories

Having something like $out/lib/gwenhywfar/plugins/60 when on Nix is a
bit pointless, because we won't have something like a central plugin
directory which could contain plugins for older library versions.

Another reason to strip the effective shared object version is that we
really want to avoid the need to track and update the SO version on
every single update.

This makes it way easier to add support for libchipcard and/or other
related packages/plugins.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @cillianderoiste, @urkud