Resolve prelude redundant import warnings in nightly

[dtolnay]

New warnings in nightly since rust-lang/rust#117772. With deny(warnings), this is causing CI to fail.

error: the item `syn` is imported redundantly
  --> derive_builder_core/src/block.rs:5:11
   |
5  | use syn::{self, spanned::Spanned, Block, LitStr};
   |           ^^^^
   |
  ::: derive_builder_core/src/lib.rs:27:1
   |
27 | extern crate syn;
   | ----------------- the item `syn` is already imported here
   |
note: the lint level is defined here
  --> derive_builder_core/src/lib.rs:18:9
   |
18 | #![deny(warnings, missing_docs)]
   |         ^^^^^^^^
   = note: `#[deny(unused_imports)]` implied by `#[deny(warnings)]`

error: the item `syn` is imported redundantly
  --> derive_builder_core/src/builder.rs:6:11
   |
6  | use syn::{self, Path, TraitBound, TraitBoundModifier, TypeParamBound};
   |           ^^^^
   |
  ::: derive_builder_core/src/lib.rs:27:1
   |
27 | extern crate syn;
   | ----------------- the item `syn` is already imported here

error: the item `darling` is imported redundantly
  --> derive_builder_core/src/macro_options/darling_opts.rs:6:15
   |
6  | use darling::{self, Error, FromMeta};
   |               ^^^^
   |
  ::: derive_builder_core/src/lib.rs:22:1
   |
22 | extern crate darling;
   | --------------------- the item `darling` is already imported here

error: the item `syn` is imported redundantly
  --> derive_builder_core/src/macro_options/darling_opts.rs:9:11
   |
9  | use syn::{self, spanned::Spanned, Attribute, Generics, Ident, Path};
   |           ^^^^
   |
  ::: derive_builder_core/src/lib.rs:27:1
   |
27 | extern crate syn;
   | ----------------- the item `syn` is already imported here