package: consolidate get_packages logic

Refactor get_package logic.
With this refactoring, code like
"if is_ubuntu; then install_package xxx elif is_fedora..."
can be simplified later.

Change-Id: I489bfd4cc12cc6b0b8201837f2bfb78c6881c82c
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Merge "package: consolidate get_packages logic"