This is the part of the rails source base (looted from /opt/local/lib/ruby/gems/1.8/gems/rails-2.2.2/lib/rails_generator/lookup.rb )
that handles going out to look for generators in rubygems.  I will need to do something like this so that people can write goals for
goaloc.  But not yet.  At first I'll just build them in.  I might want to rip off the code generator from rails, also, or perhaps
build my own.  


    class AbstractGemSource < Source
      def initialize
        super :RubyGems
      end
    end

    # GemGeneratorSource hits the mines to quarry for generators.  The latest versions
    # of gems named *_generator are selected.
    class GemGeneratorSource < AbstractGemSource
      # Yield latest versions of generator gems.
      def each
        dependency = Gem::Dependency.new(/_generator$/, Gem::Requirement.default)
        Gem::cache.search(dependency).inject({}) { |latest, gem|
          hem = latest[gem.name]
          latest[gem.name] = gem if hem.nil? or gem.version > hem.version
          latest
        }.values.each { |gem|
          yield Spec.new(gem.name.sub(/_generator$/, ''), gem.full_gem_path, label)
        }
      end
    end

    # GemPathSource looks for generators within any RubyGem's /rails_generators/<generator_name>_generator.rb file.
    class GemPathSource < AbstractGemSource
      # Yield each generator within rails_generator subdirectories.
      def each
        generator_full_paths.each do |generator|
          yield Spec.new(File.basename(generator).sub(/_generator.rb$/, ''), File.dirname(generator), label)
        end
      end

      private
        def generator_full_paths
          @generator_full_paths ||=
            Gem::cache.inject({}) do |latest, name_gem|
              name, gem = name_gem
              hem = latest[gem.name]
              latest[gem.name] = gem if hem.nil? or gem.version > hem.version
              latest
            end.values.inject([]) do |mem, gem|
              Dir[gem.full_gem_path + '/{rails_,}generators/**/*_generator.rb'].each do |generator|
                mem << generator
              end
              mem
            end
        end
    end