Support multiple crate versions in --extern-html-root-url
Rustdoc's `--extern-html-root-url` used to use `tcx.crate_name()` to identify crates, but that used crates' internal names from their metadata, instead of names given to them in `--extern`. That was confusing, because both `--extern…` arguments seem related and use similar syntax. Crucially, this didn't work correctly with Cargo's package aliases or multiple versions of crates.
`sess.opts.externs` lacks `CrateNum`, and `Resolver.extern_prelude` gets destroyed before `rustdoc` has a chance to see it, so I've had to save this mapping in `CStore`.
Just in case, I've kept the previous mapping by crate name as a fallback for crates that weren't matched by their extern name.
Fixesrust-lang/rust#76296
b38ece9021