Rollup merge of #141989 - aDotInTheVoid:sin-sooner, r=GuillaumeGomez

rustdoc-json-type: Depend on `serde` and `serde_derive` seperately

Before this commit, serde_derive is built before serde. But serde does not depend on serde_derive, so that is not needed. Instead, build serde and serde_derive in parallel.

This speeds up compilation for users depending on rustdoc-json-types out of tree.

Imports: rust-lang/rustdoc-types#49

CC ``@Enselic``

r? ``@GuillaumeGomez``

Author: matthiaskrgr