[Resolver] Compare against locked dependency upfront

This adds a forward checking step that proves to be a major optimization when there are unresolvable conflicts involving the Lockfile

Thanks to @pietbrauer

CHANGELOG.md

@@ -6,6 +6,12 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
 
 ## Master
 
+##### Enhancements
+
+* Allow the resolver to fail faster when there are unresolvable conflicts
+  involving the Lockfile.  
+  [Samuel Giddins](https://github.com/segiddins)
+
 ##### Bug Fixes
 
 * Allows pre-release spec versions when a requirement has an external source

lib/cocoapods/resolver.rb

@@ -104,9 +104,10 @@ module Pod
     def search_for(dependency)
       @search ||= {}
       @search[dependency] ||= begin
+        requirement = Requirement.new(dependency.requirement.as_list << requirement_for_locked_pod_named(dependency.name))
         specs = find_cached_set(dependency).
           all_specifications.
-          select { |s| dependency.requirement.satisfied_by? s.version }.
+          select { |s| requirement.satisfied_by? s.version }.
           map { |s| s.subspec_by_name(dependency.name, false) }.
           compact
 
@@ -313,6 +314,16 @@ module Pod
       cached_sets[name]
     end
 
+    # @return [Requirement, Nil]
+    #         The {Requirement} that locks the dependency with name `name` in
+    #         {#locked_dependencies}.
+    #
+    def requirement_for_locked_pod_named(name)
+      if vertex = locked_dependencies.vertex_named(name)
+        vertex.payload.requirement
+      end
+    end
+
     # @return [Set] Creates a set for the Pod of the given dependency from the
     #         sources. The set will contain all versions from all sources that
     #         include the Pod.