This refactors the importer abstraction to use a shared
`Insertion`. This is mostly just moving some code around
with some slight tweaks.
The plan here is to keep the rest of the importing code
in `ruff_linter` and then write something ty-specific on
top of `Insertion`. This ends up sharing some code, but
not as much as would be ideal. In particular, the
`ruff_linter` imported is pretty tightly coupled with
ruff's semantic model. So to share the code, we'd need to
abstract over that.
