[LazySpecification] Warn when there are multiple sources per version in #all_specifications

Addresses #3 of https://github.com/CocoaPods/CocoaPods/pull/2637#issuecomment-58584402

lib/cocoapods/resolver/lazy_specification.rb

 module Pod
-  class Specification::Set
+  class Specification
+    class Set
       class LazySpecification < BasicObject
         attr_reader :name, :version, :source
 
@@ -33,9 +34,22 @@ module Pod
       end
 
       def all_specifications
-      @all_specifications ||= versions_by_source.map do |source, versions|
+        @all_specifications ||= begin
+          versions_by_source.reduce({}) do |sources_by_version, (source, versions)|
+            versions.each { |v| (sources_by_version[v] ||= []) << source }
+            sources_by_version
+          end.select { |_version, sources| sources.count > 1 }.each do |version, sources|
+            UI.warn "Found multiple specifications for `#{name} (#{version})`:\n" +
+              sources.
+                map { |s| s.specification_path(name, version) }.
+                map { |v| "- #{v}" }.join("\n")
+          end
+
+          versions_by_source.map do |source, versions|
             versions.map { |version| LazySpecification.new(name, version, source) }
           end.flatten
         end
       end
+    end
+  end
 end