POC: Bulk rust import scanning

[Peter554]

A sketch of bulk import scanning via rust.

Ideas to make this nicer:

• This change makes the existing ImportScanner python interface obsolete. We should likely instead introduce a BulkImportScanner interface, which processes all modules at once.
• Many tests fail, mainly because rust does not respect the mock/stub filesystem. I'm unsure what to do about this / how much I care about this (the added snapshot test gives me a lot of confidence in the change, so maybe those older tests can just be deleted).

This seems to make import scanning a little bit faster on my machine, but the change doesn't seem to be really big.

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #222 will degrade performances by 77.57%

_{Comparing Peter554:bulk-rust-import-scanning (f0d4871) with master (ce0d32f)}

Summary

⚡ 3 improvements
❌ 2 regressions
✅ 17 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_deep_layers_large_graph_kept	19.8 ms	17.2 ms	+15.19%
⚡	test_no_chain	1.2 ms	1.1 ms	+12.45%
⚡	test_no_chains	1.2 ms	1.1 ms	+12.43%
❌	test_build_django_from_cache_a_few_misses[350]	298.4 ms	726.3 ms	-58.92%
❌	test_build_django_uncached	145.2 ms	647.5 ms	-77.57%

[Peter554]

Closing this one since we're working on it in other PRs