Fix packager dep

[jreidinger]

Problem

pull request https://github.com/yast/yast-yast2/pull/1253 cause several build failures when module even transitively need SlideShow, but does not need packager to build itself.

Solution

Load packager library only dynamic and handle if it is not available.

Testing

• adapted unit tests to demonstrate that mocking of packager is not needed

Screenshots

not affected

[yast-bot]

:heavy_check_mark: Internal Jenkins job #6 successfully finished :heavy_check_mark: Created IBS submit request #271708