App.is_excluded_from_build() uses the wrong function entirely to
query the [build.exclude] section of project.conf
(App.get_project_refs() instead of App.get_value()). This has
obviously never worked. It rose to prominence because commit 6db73873
introduced App.__proj_dir(), which now raises an Exception if passed
garbage, which in turn surfaces as
Exception: No project path found for module "debian"
Use the correct function for that: App.get_value().
Signed-off-by: Jan Lindemann <jan@janware.com>