Remove garbage files from staging dir before packaging

[uroboro]

Theos could delete files matching the THEOS_RSYNC_EXCLUDES list (common.mk#L269). This would ensure that files that are ignored during copy with rsync also do not exist by accident (like with .DS_Store, which somehow people manage to ship onto repositories).

This could be done as an unconditional step in the internal-package:: (package.mk#L9).