Pass also filename to Dpkg::Changelog::Parse::changelog_parse()

With the upcoming dpkg 1.23.0, the "file" option is deprecated and emits a warning to that effect, which is replaced by the "filename" option.

To ease in the migration and to avoid making code not backportable, the deprecated option is accepted with no warning if the new option is passed as well, so that both old and new dpkg modules can be used. Only after 1.24.0 the old option will be obsoleted and unconditionally start generating warnings or errors.