common.mk: separate Unicode files directory

* common.mk: download Unicode files into separate directory for the
  version.

* tool/downloader.rb (Downloader::Unicode.download): strip version
  directory.

* tool/downloader.rb (Downloader.download): strip directory name
  if download directory is given.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48196 b2dd03c8-39d4-4d8f-98ff-823fe69b080e