rustdoc search: path matches that skip segments should be deprioritized

[lolbinarycat]

a search for collections::b matches std::collections::LinkedList::back far before it matches std::collections::BTreeMap.

ommitting segments at the start and end should not be penalized, but ommitting them in the middle should be.

[lolbinarycat]

@rustbot label A-rustdoc-search T-rustdoc