Fix issues related to running xdist with the terminal plugin disabled

This fixes an issue where the pytest plugin manager returns None if a plugin is not loaded instead of raising an error.
It also makes terminal optional in plugin code.

src/xdist/plugin.py

@@ -170,7 +170,8 @@ def pytest_configure(config):
         session = DSession(config)
         config.pluginmanager.register(session, "dsession")
         tr = config.pluginmanager.getplugin("terminalreporter")
-        tr.showfspath = False
+        if tr:
+            tr.showfspath = False
     if config.getoption("boxed"):
         config.option.forked = True