[Installer] Cache locally sourced pods.

The caching of LocalPods might need a refactor.

Closes #485

lib/cocoapods/installer.rb

@@ -190,7 +190,7 @@ module Pod
         @pods_by_spec[target_definition.platform] = {}
         result[target_definition] = specs.map do |spec|
           if spec.local?
-            LocalPod::LocalSourcedPod.new(spec, sandbox, target_definition.platform)
+            @sandbox.locally_sourced_pod_for_spec(spec, target_definition.platform)
           else
             @sandbox.local_pod_for_spec(spec, target_definition.platform)
           end

lib/cocoapods/sandbox.rb

@@ -14,6 +14,7 @@ module Pod
       @build_headers = HeadersDirectory.new(self, BUILD_HEADERS_DIR)
       @public_headers = HeadersDirectory.new(self, PUBLIC_HEADERS_DIR)
       @cached_local_pods = {}
+      @cached_locally_sourced_pods = {}
       FileUtils.mkdir_p(@root)
     end
 
@@ -37,6 +38,15 @@ module Pod
       end
     end
 
+    # TODO: refactor the pods from a local source should not be chached by the sandbox
+    #
+    def locally_sourced_pod_for_spec(spec, platform)
+      key = [spec.top_level_parent.name, platform.to_sym]
+      (@cached_locally_sourced_pods[key] ||= LocalPod::LocalSourcedPod.new(spec.top_level_parent, self, platform)).tap do |pod|
+        pod.add_specification(spec)
+      end
+    end
+
     def installed_pod_named(name, platform)
       if spec_path = podspec_for_name(name)
         key = [name, platform.to_sym]