update docstring

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
change return of AptArchive's methods

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
add unittest and refactoring

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>