Apply splat to schema sets and path aliases without explicit select. (#929)

Ensures a splat is applied in the following cases:
```py
default.User
default.User.filter(...)
default.User.groups
default.User.groups.filter(...)
default.User.filter(...).groups
default.User.filter(...).groups.filter(...)
```

Also ensures that splats are applied if such a case is used within another object's select.
```py
default.Group.select(
    users=default.Post.author  # splat applied here
)
```