Resolve RT#64208, solving the issue of duplicate paths ONCE AND FOR ALL.

Resolve RT#64208, solving the issue of duplicate paths ONCE AND FOR ALL.

If asked to add a path that is already the most recent path activated,
local::lib won't try to doanything. If asked to add a path that's
already been activated further down the stack, it will remove it from
its current position and re-add, thus making it the new "most recent
path" without duplication. In other cases, everything is the same.

This adds yet another variable to pass down through three layers of
method calls. Something must be done about those.