[pkg] Drop "support" of unused subpackage_of option in setup.py __pkginfo__ does not declare subpackage_of.

Add missing import, eaten by previous merge

Only access once to _cw.message in the login form else the first access may pop the message and the second won't get anything to display. This works on default message implementation but doesn't on the pyramid's one, leading to no message displayed on erroneous authentication attempt, which is not very user friendly.