Show feature resolver differences for dev-dependencies.

[ehuss]

During the crater run for 2021, there was a package that failed to update in a confusing way. The issue is that a feature was removed in the new resolver, but only for a dev-dependency. The report displayed with cargo fix --edition did not say anything about that, so it took me a bit to figure it out. This changes it so that the report also includes changes to features of dev-dependencies. I honestly don't remember my thought process behind the original code.

For example, the offending package now says:

When building the following dependencies, the given features will no longer be used:

  log v0.4.8 removed features: std
  syn v0.15.44 (as host dependency) removed features: extra-traits, visit

The following differences only apply when building with dev-dependencies:

  phf_shared v0.7.24 (as host dependency) removed features: unicase

And the error that happens after updating to 2021 is:

error[E0277]: the trait bound `UniCase<&str>: phf_shared::PhfHash` is not satisfied
  --> /Users/eric/.cargo/registry/src/github.heygears.com-1ecc6299db9ec823/mime_guess-1.8.7/build.rs:37:21
   |
37 |         forward_map.entry(UniCase(key), &format!("{:?}", val));
   |                     ^^^^^ the trait `phf_shared::PhfHash` is not implemented for `UniCase<&str>`

Hopefully developers will be able to see the note about the feature unicase being removed from phf_shared, and the error message about UniCase not implementing PhfHash, and connect the two together. Previously, the upgrade report didn't mention anything about phf_shared, and thus no clues on what went wrong.

[rust-highfive]

r? @Eh2406

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[alexcrichton]

@bors: r+

[bors]

📌 Commit 5939e21 has been approved by alexcrichton

[bors]

⌛ Testing commit 5939e21 with merge 259ea8c...

[bors]

☀️ Test successful - checks-actions
Approved by: alexcrichton
Pushing 259ea8c to master...