add support for extra cmake finders

Summary:
this allows us to find dependencies as proper packages rather than
simulate packages and in turn bypass cmake's feature summary and rendering
the dependencies listed in a non-standard fashion

Test Plan: still configures

Reviewers: apol

Reviewed By: apol

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D20807