[corsonknowles]

Stacked on #60. This branch contains #60's commit (1df54a4) plus one more, 83eba89. Please review only 83eba89 here; once #60 merges, this PR's diff shrinks to just that commit.

What this adds

tests/auto_correct_unused_dependencies_flows_test.rs runs check-unused-dependencies --auto-correct end to end against small projects built in a temporary directory. Each project has four library packs that define modules, plus consumer packs whose code references some of them, so whether a dependency is "unused" comes from real references, not stubs.

Every test snapshots every file in the project before and after the run and compares them whole. A test therefore fails when an intended edit is missing, and also when any other line or any other file changes. After each correction, the project must pass check-unused-dependencies, check and validate.

Flows

Test	Proves
removes_one_unused_dependency_from_a_documented_package_yml	The packwerk init / pks create template, with comments filled in, loses one entry and nothing else. A comment that still covers a kept entry stays.
removes_a_comment_with_the_only_entry_it_covers	A comment separated by blank lines goes with its only entry.
edits_every_pack_with_unused_dependencies_and_no_other_file	Several packs in one run: each is edited, and an already-tidy pack is not touched.
a_second_run_changes_nothing	Running again is a no-op.
removing_the_last_dependency_keeps_the_key_while_comments_remain	A commented-out entry and a comment for the next key that contains a - line survive. The key stays.
removing_the_last_dependency_removes_an_uncommented_block	With no comments, the block goes, as serializing would do.
removing_dependencies_over_several_runs_matches_one_run	Dropping usages one at a time over four runs gives the same file at every step as one run from the original.
leaves_other_lists_that_name_the_same_packs_alone	ignored_dependencies, visible_to and a nested metadata.dependencies naming the removed pack are untouched.
preserves_crlf_line_endings_and_a_missing_final_newline	Line endings and the lack of a trailing newline are kept.
falls_back_to_rewriting_only_the_files_it_cannot_edit	A flow-style list is rewritten with a warning naming the removed dependencies in sorted order, while a block list in the same run is still edited in place.
without_auto_correct_reports_and_writes_nothing	A plain check writes nothing.
never_removes_a_dependency_that_is_used	Plain, double-quoted and single-quoted entries that are all used are left alone.

Checking the tests bite

• Against main's auto-correct, which serializes the pack, 10 of the 12 fail. The two that pass are the no-write and all-used guards, which should pass with or without Preserve comments when auto-correcting unused dependencies #60.
• With Preserve comments when auto-correcting unused dependencies #60's pre-review code (2c5ee4f) in place of dependency_removal.rs and checker.rs, exactly the two flows from the last review fail: emptying a list that has comments, and removing over several runs.

cargo test (including with RUST_LOG=debug set), cargo clippy --all-targets -- -D warnings and cargo fmt --check pass.

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