Don't require package names in pillar data

[noelmcloughlin]

The pillar.example illustrates a design problem where, in some scenarios, package names must be defined as pillars. This requirement should be removed or replaced by abstract flags-

• require_contrib_package = True
• require_python_package = True

Furthermore, in my opinion, if pillar data includes postgres.extension list then contrib package should be auto-installed. Formula user should not need to know packaging implementation, only how to model their requirements as pillars.