find_spec_by_name_and_version

# File lib/rubygems/dependency_installer.rb, line 264
  def find_spec_by_name_and_version gem_name,
                                    version = Gem::Requirement.default,
                                    prerelease = false
    set = Gem::AvailableSet.new

    if consider_local?
      if gem_name =~ /\.gem$/ and File.file? gem_name then
        src = Gem::Source::SpecificFile.new(gem_name)
        set.add src.spec, src
      elsif gem_name =~ /\.gem$/ then
        Dir[gem_name].each do |name|
          begin
            src = Gem::Source::SpecificFile.new name
            set.add src.spec, src
          rescue Gem::Package::FormatError
          end
        end
      else
        local = Gem::Source::Local.new

        if s = local.find_gem(gem_name, version)
          set.add s, local
        end
      end
    end

    if set.empty?
      dep = Gem::Dependency.new gem_name, version
      dep.prerelease = true if prerelease

      set = find_gems_with_sources(dep, true)
      set.match_platform!
    end

    if set.empty?
      raise Gem::SpecificGemNotFoundException.new(gem_name, version, @errors)
    end

    @available = set
  end