Docs.rs is generating external dependency links

[Nemo157]

The source link for https://docs.rs/rand/latest/rand/trait.SeedableRng.html#method.seed_from_u64 links to https://rust-random.github.io/rand/src/rand_core/lib.rs.html#306 instead of https://docs.rs/rand_core/0.6.3/src/rand_core/lib.rs.html#306-335; and https://docs.rs/rand_mt/4.1.1/rand_mt/struct.Mt19937GenRand32.html#impl-RngCore links to https://rust-random.github.io/rand/rand_core/trait.RngCore.html instead of https://docs.rs/rand_core/0.6.3/rand_core/trait.RngCore.html.

IIRC there was some plan on changing how --extern-html-root-url worked when a crate specified its own html_root_url, but there was going to be a flag for docs.rs to keep the previous override behavior.

[jyn514]

This is #1473, I haven't had time to work on it. Happy for someone else to fix it up - I think you may just need to rerun the ci job?