Stop requiring `ext/debian` be packaged in the sources

This syncs the behaviour of the Debian package builder to the RPM builder: now
it will install `ext/debian` from the outside of the package, rather than
requiring that you list it in your project file list -- and then carefully
destroy it during the build of, eg, any other package format.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>