Change install_packages semantics

Currently all items per action, e.g aptitude, install aptitude-r
etc., are collected and executed in the predefined @command
sequence. This completely ignores the class order, which may result
in broken installation.

This especially happens in our case of the Ubuntu HWE, where our
main system packages are installed with aptitude-r, including the
correct HWE dependencies, but any aptitude class is always run
before the aptitude-r class. This results in broken installations.
The two non-obvious ways for the user to fix this problem is either
to include the HWE dependencies in his class or use the same install
method, which might not be acceptable.

This patch completely changes the install semantics by taking the
class list into account and dropping the predefined action list.
It generates a list of all single actions in the class order and
just merges consecutive equal action items.