[Target] Move #target_definition to AggregateTarget

lib/cocoapods/target.rb

@@ -6,11 +6,6 @@ module Pod
   # This class is used to represent both the targets and their libraries.
   #
   class Target
-    # @return [TargetDefinition] the target definition of the Podfile that
-    #         generated this target.
-    #
-    attr_reader :target_definition
-
     # @return [Sandbox] The sandbox where the Pods should be installed.
     #
     attr_reader :sandbox

lib/cocoapods/target/aggregate_target.rb

@@ -3,6 +3,10 @@ module Pod
   # of the single Pods. The client targets will then depend on this one.
   #
   class AggregateTarget < Target
+    # @return [TargetDefinition] the target definition of the Podfile that
+    #         generated this target.
+    attr_reader :target_definition
+
     # Initialize a new instance
     #
     # @param [TargetDefinition] target_definition @see target_definition