Auto merge of #3781 - alexcrichton:beta-backport, r=alexcrichton

Auto merge of #3781 - alexcrichton:beta-backport, r=alexcrichton

[backport] Fix deps with `cargo test --all` and doctests

This commit fixes `cargo test --all` with the way we ship libraries to `rustdoc
--test`. I'm... not entirely sure what the previous incarnation was doing but
the current organization is to interpret `compilation.libraries` as a mapping
from a package to the list of crates it needs to link to test.

This updates the support in `cargo_rustc/mod.rs` to create the map appropriately
and tweaks the loop in `cargo_test.rs` as well.

Closes rust-lang/rust#39879