Basically, given a `from module import name1, name2, ...` statement,
we'd like to be able to insert another name in that list.
This new `Insertion::existing_import` API provides such
functionality. There isn't much to it, although we are careful
to try and avoid inserting nonsense for import statements
that are already invalid.
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.
