search our cmake dir first

Summary:
if we search our dir last we risk not being forwards compatible with ECM.

specific current example: FindCanberra is now also in ECM, if that didn't
have specific backwards compatibility variables for KDE software it'd
not be compatible with our FindCanberra and break the buld. as such we
must make sure that when we have a finder it is preferred over ECM.

Reviewers: apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19381