Bug 1730712: Ensure virtualenv up-to-date before activation r=ahal

Rather than requiring that consumers remember to `ensure()` before
calling `activate()`, we can do so automatically during activation.
There isn't a valid use case in which we want obsolete virtualenvs to be
activateable.

Usages of `MozSiteManager` have been updated accordingly.

Differential Revision: https://phabricator.services.mozilla.com/D129688

python/mach_commands.py

@@ -84,9 +84,10 @@ def python(
             requirements.pths_as_absolute(command_context.topsrcdir)
         )
     else:
-        command_context.virtualenv_manager.ensure()
         if not no_activate:
             command_context.virtualenv_manager.activate()
+        else:
+            command_context.virtualenv_manager.ensure()
         python_path = command_context.virtualenv_manager.python_path
         if requirements:
             command_context.virtualenv_manager.install_pip_requirements(