Use `Importer` available on `Checker` (#11513)
## Summary
This PR updates the `FA102` rule logic to use the `Importer` which is
available on the `Checker`.
The main motivation is that this would make updating the `Importer` to
use the `Tokens` struct which will be required to remove the
`lex_starts_at` usage in `Insertion::start_of_block` method.
## Test Plan
`cargo insta test`
