clean up kpack_lifecycle and get k8s client calls out of constructor

[#171029528]

Co-authored-by: Connor Braa <cbraa@pivotal.io>
Co-authored-by: Matt Royal <mroyal@pivotal.io>

Author: cwlbraa

lib/cloud_controller/diego/lifecycles/kpack_lifecycle.rb

@@ -2,17 +2,11 @@
 
 module VCAP::CloudController
   class KpackLifecycle
-    attr_reader :staging_message, :buildpack_infos
+    attr_reader :staging_message
 
     def initialize(package, staging_message)
       @staging_message = staging_message
       @package = package
-
-      @buildpack_infos = requested_and_available_buildpacks(buildpacks_to_use)
-      @validator = KpackLifecycleDataValidator.new(
-        requested_buildpacks: buildpacks_to_use,
-        buildpack_infos: buildpack_infos
-      )
     end
 
     delegate :valid?, :errors, to: :validator
@@ -36,7 +30,16 @@ def stack
       nil
     end
 
-    attr_reader :validator
+    def buildpack_infos
+      @buildpack_infos ||= requested_and_available_buildpacks(buildpacks_to_use)
+    end
+
+    def validator
+      @validator ||= KpackLifecycleDataValidator.new(
+        requested_buildpacks: buildpacks_to_use,
+        buildpack_infos: buildpack_infos
+      )
+    end
 
     private
 
@@ -49,11 +52,9 @@ def buildpacks_to_use
     end
 
     def requested_and_available_buildpacks(buildpacks_to_use)
-      # TODO: extract a common way to get the unfiltered list of buildpacks that can then have filters applied if needed?
-      # TODO: Don't reach out to k8s unless buildpacks are requested
-      available_buildpack_names = KpackBuildpackListFetcher.new.fetch_all.map(&:name)
+      return [] if buildpacks_to_use.empty?
 
-      buildpacks_to_use & available_buildpack_names
+      buildpacks_to_use & KpackBuildpackListFetcher.new.fetch_all.map(&:name)
     end
   end
 end