package_manager: Share more common DEB / IPK code

Avoid code duplication by making `extract` a shared method (and
retrieving the package manager specific input via an abstract method).
Additionally, follow Python conventions and prefix class internal
methods with "_" to indicate that they shouldn't be called externally.

Signed-off-by: Philip Lorenz <philip.lorenz@bmw.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>