Don't use ports and adapters for ImportGraph

[seddonym]

It's never brought any value to have an abstraction for the ImportGraph. So this removes the abstract class and then moves the concretion to the application layer.

[ ] Add tests for the change. In general, aim for full test coverage at the Python level. Rust tests are optional.
[ ] Add any appropriate documentation.
[ ] Add a summary of changes to CHANGELOG.rst.
[ ] Add your name to AUTHORS.rst.
[ ] Run just full-check.

[codspeed-hq]

CodSpeed Performance Report

Merging #254 will not alter performance

_{Comparing no-dip-graph (a279485) with main (8ddc6fb)}

Summary

✅ 23 untouched
⏩ 21 skipped¹

Footnotes

1. 21 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩