Fix 'convert_path()' so it returns immediately under Unix -- prevents blowing up when the pathname starts with '/', which is needed when converting installation directories in the "install" command.
Fix 'convert_path()' so it returns immediately under Unix -- prevents blowing
up when the pathname starts with '/', which is needed when converting
installation directories in the "install" command.

pypa/setuptools,pypa/setuptools,pypa/setuptools