Add module expressions

[seddonym]

Adds module expressions - most of the work is from #188, with a few tweaks on top.

[Peter554]

Consider whether the Python API for import expressions should use two-tuples or arrow-strings (i.e. "importer -> imported").

@seddonym Incase it gets buried: x-posting my remaining concern about the arrow strings here -> #188 (comment). I'm fine with whatever decision you want to take here though.

[codspeed-hq]

CodSpeed Performance Report

Merging #192 will improve performances by 14.25%

_{Comparing add-module-expressions (48053df) with master (8c1bb5d)}

Summary

⚡ 4 improvements
✅ 13 untouched benchmarks
🆕 2 new benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_deep_layers_large_graph_kept	19.3 ms	16.9 ms	+14%
⚡	test_deep_layers_large_graph_violated	11.9 ms	10.4 ms	+14.25%
⚡	test_no_chain	1.2 ms	1.1 ms	+12.24%
⚡	test_no_chains	1.2 ms	1.1 ms	+12.22%
🆕	test_find_matching_direct_imports	N/A	37.4 ms	N/A
🆕	test_find_matching_modules	N/A	9.5 ms	N/A

[Peter554]

Should be Ok(Regex::new(&(r"^".to_owned() + &pattern_parts.join(r"\.") + r"$")).unwrap()) i.e. \. instead of .. Sorry, this one slipped past me 🙈

[Peter554]

Thanks for finishing this off, and for finding that bug!