Attach to Process is broken when old version of ptvsd is in site-packages


When importing repl and util modules from the main debugger script, it tries to import them from ptvsd package first and foremost. If ptvsd is installed in site-packages of the current active interpreter or venv, then that import succeeds, and the module from that package is used instead of loading the code from install directory. If the version of ptvsd that is installed in site-packages does not match PTVS version, it breaks attach completely.