Fix: Valid plugins are marked as Invalid

This is a regression introduced in c12bf0aa1e2 where it requires all
files in $searches to be present. Instead, only mark it as invalid
when none of those files can be found.

Reported-By: Mohammad Saleh
Ref: http://thread.gmane.org/gmane.comp.cms.croogo/191
Reuse $path value
Merge branch '1.5-extensions-fix'