[gazelle] Support -index=lazy

Gazelle supports `-index=lazy` to speed up indexing directories. This currently fails on Python with a bunch of errors like
```
my_project/foo/bar.py: "foo.other.baz" is an invalid dependency
```

Perhaps a `gazelle:py_search` directive is needed?

Reproduce the failure in Gazelle with Python and `-index=lazy`, focusing on the reported invalid dependency errors such as `foo.other.baz`. Investigate whether a `gazelle:py_search` directive is needed; done means lazy indexing works for Python without those dependency errors.

索引モデルが issue の本文から書いたものです。