add: PasswordChecker interface

add: PasswordChecker interface

PasswordChecker is a generic version of the interface which
PasswordStrengthChecker currently offers. PasswordStrengthChecker is
changed to implement the new interface (currently the only
implementation, but others will be added in the future).

Using this interface instead of PasswordStrengthChecker directly in
other code will let us introduce other ways of checking password
strength as well, e. g. implementing #143.

The interface is placed in the new `passwords` subpackage, and the
PasswordStrengthChecker implementation is also moved there.

Change-Id: I2fb9dde216db7b14f3d4d45342bdc5c657c87233