extension points shouldn't depend on implementation detail class, so promoted ComputerImpl to SlaveComputer.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@9402 71c3de6d-444a-0410-be80-ed276b4c234a

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@9395 71c3de6d-444a-0410-be80-ed276b4c234a

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@9394 71c3de6d-444a-0410-be80-ed276b4c234a