Switch the pkpass formatter plugin to use KPkPass

Summary:
This brings no functional change, but it makes the pkpass handling code
reusable in other places too.

Reviewers: KDE PIM, mlaurent, knauss

Reviewed By: KDE PIM, mlaurent, knauss

Subscribers: mlaurent, knauss

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D11830