builder: Move runner logic in main into its own function
builder: Move runner logic in main into its own function

Signed-off-by: Yechiel Kalmenson <b570fba9aecf1651fa1bd75345294fe6019b6e96@pivotal.io>

cloudfoundry-incubator/buildpack_app_lifecycle,cloudfoundry-incubator/buildpack_app_lifecycle