rustdoc: Fix LinkReplacer link matching
It currently just uses the first link with the same href which might not necessarily be the matching one.
This fixes replacements when there are several links to the same item but with different text (e.g. `[X] and [struct@X]`). It also fixes replacements in summaries since those use a links list with empty hrefs, so currently all links would always match the first link by href but then not match its text. This could also lead to a panic in the `original_lext[1..len() - 1]` part when the first link only has a single character, which is why the new code uses `.get(..)` instead.
This commit makes intra-doc link tooltips consistent with generated
links in function signatures and item tables, with the format
`itemtype foo::bar::baz`. This way, you can tell if a link points at
a trait or a type (for example) by mousing over it.
See also fce944d4e7
