As far as I can see this is a bug in the apache2 packaging. The httpd
virtual package should be provided by the apache2 package, not the
apache2-bin package, because the apache2-bin package doesn't provide a
working webserver. Bug report I just filed about this:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=756361
A policy bug about what httpd-wsgi should provide is still open however:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=588497
It isn't very easy to specify what httpd-wsgi should mean, because
some WSGI servers are full webservers with WSGI support such as apache
and others like gunicorn are only supposed to run behind a proxying
server such as apache or nginx (similar to for example php-fpm).
So as far as I can see, the correct dependency should be:
Depends: libapache2-mod-wsgi | httpd-wsgi, apache2 | httpd
So it also possible to run other webservers.