[rustdoc search] allow setting crates to search in before showing results #129537
Description
specifically, this would mean moving the #crate-search dropdown so it is available outside of the results screen.
on desktops, it could be moved into the <rustdoc-search> search form, and displayed in a single line. on mobile, it could be displayed directly underneath the search bar line.
this would significantly speed up searching through the nightly-rustc docs when you already know what crate you want to search, although it would require some small refactorings of rustdoc's search.js.