DebianBuildManager.gatherResults belongs in BinaryPackageBuildManager

[Jeroen T. Vermeulen]

DebianBuildManager's implementation of gatherResults, through its helper getChangesFilename, depends on self._dscfile which is created in BinaryPackageBuildManager.initiate. Unless they set the same variable, which would be nasty, any attempt to use the base gatherResults implementation in other subclasses is doomed.