Some projects have nested setup.py files other have multiple setup.py
files. All these projects are now supported.

tox is helpful when executing tests in varying development enviroments,
but when we run our tests in CI we primarily want to ensure that our
software runs on our target distribution which are a variety of Debian
releases.

This means that the test dependencies need to be installed from Debian
itself and not by tox. We already did some groundwork for this in the
`deb.gitlab-ci.yml` which triggers a `.pre`-stage job that generates a
build dependency package that is either auto-generated from
`debian/control` or created by parsing the `DPKG_BUILD_DEPENDENCIES`
variable as a comma-separated list of packages.

If the `BUILD_DEB_PKG` variable is defined (which is taken care of by
the `build::deb-build-package` job) and the `REQUIRES_DEBIAN_BUILD_DEPS`
is != `0` (false for tests, true for linting because no actual
application code is executed) the build dependency package defined in
the `BUILD_DEB_PKG` variable will automatically installed.

Overwriting DEBBUILD_DEFAULT is weird, because we have switched to a
global include so DEBBUILD_DEFAULT would be overwritten in any case.

It’s hard to run any additional logic in jobs that extend the template
jobs, when before_script is already used. Projects can now use
before_script to do any upfront work they require.

The gitlab recipes follow my default project layout and conventions.
Though the templates and jobs usually loosely fit into a category (like
deb packaging, or python something) there are some interdependencies
between them, that weren’t clear until now. Merging them as a single
template include should underline the generic nature of these recipes.

This should fix issues with missing .orig.tar.gz files during deb
uploads that were symlinks.