A bevy of subtle fixes. Primarily #597750 because we no longer 'do' site id. Also fixed restore which left you with a broken site, and handled the fact that
the drush_config_file function would only give us the right value when we were bootstrapped to that level.

this meant we couldnt regenerate the site drushrc.php until we'd bootstrapped to drupal_configuration, which meant during restore it would always use the old values.

Also when the restore task finished, the save_site_config malarky would end up writing the old values over the correct ones.

the singular filesystem object now.

Sync is now a method of the server class, with a helpful method associated
to the services that redirect to the service's server.

Sort of fix site import, which also verifies now. Introduce d()->site_path instead of concatenating everywhere.

had to make a nasty hack to step around drush internals because site aliases
put stuff in the options context that dont come from the command line options.

Removed the provision_service function, and introduce service handler subscriptions in the environment objects.

Instead of provision_service('type') you now use d()->service('type') to access the service object.
The context object returned by d() now manages which context object's service to return, based
on the internally managed subscriptions.

This gets rid of the path function, and introduces method chaining for the file service, which results in far cleaner code.