rustdoc search doesn't find crate names

[huonw]

E.g. searching for libc doesn't show the libc crate.

[richo]

ccing myself, will have a look at this in the next week or so.

[emberian]

cc me

[steveklabnik]

Triage: searching for libc doesn't show libc.

[mitaa]

Hmm, I deliberately removed the search-index entry in #30504 because it was broken.