test(multipart,derive): move trybuild tests

[JohnTitor]

PR Type

PR_TYPE

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt.
• (Team) Label with affected crates and semver status.

Overview

To avoid cyclic dependency tree.

[JohnTitor]

@robjtede By the way, could you add me as owner of multipart-derive?

[robjtede]

@JohnTitor 👍🏻 added the core team as owners

[robjtede]

i dont love this solution, disabling doctests etc, and i think there's a better way of avoiding it during publishes, testing now...

[JohnTitor]

@robjtede
In practice, I don't think it makes a big difference though. If the derive crate is meant to be used along with the base crate, that doc should be live there, I think. How about moving docs to the main crate and putting a link on the derive side?