[TargetInstaller] Move #target_definition to AggregateTargetInstaller

lib/cocoapods/installer/target_installer.rb

@@ -184,12 +184,6 @@ module Pod
         sandbox.project
       end
 
-      # @return [TargetDefinition] the target definition of the library.
-      #
-      def target_definition
-        target.target_definition
-      end
-
       # @return [PBXGroup] the group where the file references to the support
       #         files should be stored.
       #

lib/cocoapods/installer/target_installer/aggregate_target_installer.rb

@@ -31,6 +31,12 @@ module Pod
 
       private
 
+      # @return [TargetDefinition] the target definition of the library.
+      #
+      def target_definition
+        target.target_definition
+      end
+
       # Ensure that vendored static frameworks and libraries are not linked
       # twice to the aggregate target, which shares the xcconfig of the user
       # target.