Refactor dependency analysis to properly handle code imports, dependency keys, and package names

[CathalMullan]

Resolves #184, Resolves #169, Resolves #93

This refactors the dependency analysis to be more robust when it comes to handling the differences between code imports, dependency keys and package names.

• Dependencies referenced through features are now correctly detected.
• The fix command can now remove [target.'cfg(...)'.dependencies] dependencies.
• Redundant ignores where the ignored package is actually in use now print a warning.
• Improved error messages on syn parsing errors.
• Added tests around hyphen vs underscore dependency naming, and cross-package renames.

Here's a before and after for the errors messages:

Before:

> cargo shear --expand
Analyzing /home/cmullan/workspace/rustls

unexpected token, expected `;`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After:

> cargo shear --expand
Analyzing /home/cmullan/workspace/rustls

Syntax error in rustls-ci-bench at line 467:30:
unexpected token, expected `;`

   464 |             let waker = noop_waker();
   465 |             let mut ctx = task::Context::from_waker(&waker);
   466 |             match {
>  467 |                         super let mut pinned = future;
   468 |                         unsafe { ::core::pin::Pin::new_unchecked(&mut pinned) }
   469 |                     }.poll(&mut ctx) {
   470 |                 Poll::Ready(result) => result,

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

(syn doesn't support parsing super let yet)

I've also manually tested this against a few real-world projects (rustls, opentelemetry), and it looks to work well.

[Boshen]

Thank you! The redundant warnings are working great in Oxc and Rolldown.