rustdoc: Get rid of `Cache::extern_locations` and calculate the info on-demand

[jyn514]

Right now, extern_locations is populated like this: https://github.com/rust-lang/rust/blob/d81d7ab203207c4110fbf1919a99345f7e9c1498/src/librustdoc/formats/cache.rs#L158-L171

All of these can be computed on-demand with only the CrateNum, TyCtxt, and extern_html_root_urls. We should do so to avoid doing more work than necessary (and taking up lots of memory).

Mentoring instructions:

1. Add functions src_root, name, and location to ExternalCrate, using the same implementation that's currently in Cache::populate.
2. Change all uses of cache.extern_location to use the functions instead.
3. Delete cache.extern_location.

Originally posted by @jyn514 in #84494 (comment)

[jyn514]

Note that this is blocked on #84494 so you can construct an ExternalCrate with only the CrateNum.

[tdelabro]

@rustbot claim

[jyn514]

The easy part of this was done in #84601, and I think the hard part is more trouble than it's worth (see #84601 (comment)).