Menu changes

[seanr]

This relatively small commit does two things:

• Use 'menu' as module name to allow item deletion.
• Add 'exists: Yes' optionally to items in the YAML to allow new items to be created as children of existing menu items not defined by these fixtures (i.e. subpages of a panel page with its own menu item).

[seanr]

We can't use path_load here because panels don't have URL aliases (the function just returns false). There may be a better way to validate the paths here.