Missing call to visitors in case no package is defined

``onReachingBuildEnd`` should be called before ``onAfterBuild`` (on visitors) when the number of packages remaining to be built is 0, so that visitors have the opportunity to define new packages.
When the initial number of packages is 0, ``onReachingBuildEnd`` was not called. This pull request fixes this issue.

This fix is useful so that ``atpackager`` can be used without any package defined, relying on ``CopyUnpackaged`` to create an output file for each source file.