[isort] Simplify code structure for ordering imports (#8685)
While fixing #8661 I noticed that the code structure for sorting imports
could be simplified.
## Summary
- Move the logic for `force_sort_within_sections` from `isort/mod.rs` to
`isort/ordering.rs` => now there is just one line in `isort/mod.rs`:
`let imports = order_imports(import_block, settings);` which yields the
sorted imports
- Change the function signature of `order_imports` to directly return a
`Vec<EitherImport<'a>>` => no need for `OrderedImportBlock`
I think this is a bit of an improvement because the code is simpler and
there should be a bit of a speedup when setting
`force-sort-within-sections` to true. Indeed, when it's set to true
we're now directly ordering all the imports, whereas before we would
first order the straight imports, then the from imports, combine them
and finally sort the combination a second time (this is probably not
noticeable in practice though).
## Test Plan
No tests added, this is a simple refactor.
