Support a well-known script (path) for building dependencies.
With this change KDE projects may inject logic for building dependencies which are not covered by normal KDE tooling as a custom shell script.
This shell script is an alternative to encoding the same information in the 'current-jobs.json' blob for binary-factory, with a few advantages:
- It is not JSON but a free formatted shell script
- It can be maintained directly by the app project
- It can be tested by the app developers locally
- It is automatically picked up by build-generic now. I.e. it actually works during local development.
In turn this means builds should "just work" when build-generic is invoked, even for projects with unconventional dependencies. This should make the life of app developers significantly easier.