Dot directories rarely contain anything we would want to load as a
dynamic plugin. Even worse, they likely contain files with externally
controlled lifetimes which might be removed while we are using them
(see e.g., zeek/btest#98).
With this patch we do not search _discovered_ dot directories anymore.
We continue to load from a user-specified `ZEEK_PLUGIN_PATH`, even if
its name starts with a dot.
Since this patch changes previous behavior it is a **BREAKING CHANGE**.
