[A-1493] sbopkg Dev.: Revisions 847, 848, 849

r819 mostly replaced directory_checks() and ck_dir() with dir_init() but forgot about the MKDIR_PROMPT variable used in ck_dir().

Applied patch by Mauro which modified get_source_names() to only attempt to ls SRCNAME if SRCNAME is not empty so that it doesn't list the entire cache directory and break get_obsolete_sources().

Modified build_package() to export whatever we're using for REPO_TAG as TAG.