[Source] Allow the specify the master repo dir from an environment variable.

The variable is `CP_MASTER_REPO_DIR` and potentially paves the way for
supporting a deeper linting of CocoaPods/Specs. Every check would be
supported, including checking that the files patterns match, except
xcodebuild checks.

lib/cocoapods/source.rb

@@ -6,6 +6,9 @@ module Pod
       end
 
       def dirs
+        if ENV['CP_MASTER_REPO_DIR']
+          [Pathname.new(ENV['CP_MASTER_REPO_DIR'])]
+        else
           repos_dir = Config.instance.repos_dir
           unless repos_dir.exist?
             raise Informative, "No spec repos found in `#{repos_dir}'. " \
@@ -13,6 +16,7 @@ module Pod
           end
           repos_dir.children.select(&:directory?)
         end
+      end
 
       def names
         dirs.map { |repo| repo.basename.to_s }.sort