Nested autocompletion

Autocompletion (tab) does work here

```julia
@pyimport numpy as np
np.linalg.
```

but doesn't here

```julia
np = pyimport("numpy")
np.linalg.
```

Start by reproducing the two Julia snippets using @pyimport and pyimport("numpy"), then compare how tab completion handles np.linalg in each case. Done means nested completion works consistently for both forms, including np.linalg..

索引モデルが issue の本文から書いたものです。