Auto merge of #5186 - infinity0:stricter-need-dev-deps, r=alexcrichton

Stricter need_dev_deps behaviour

The previous PR (#5012) contained an unnecessary work-around for behaviour of `--all-targets` that was misunderstood. This PR removes that work-around and adds some tests and comments to clarify the behaviour for future contributors, which may help to make easier a future fix for #5177 and #5178.

diff --cc src/bin/commands/rustc.rs
Simple merge

diff --cc src/bin/commands/rustdoc.rs
Simple merge

diff --cc tests/testsuite/build.rs
Simple merge

diff --cc tests/testsuite/rustc.rs
Simple merge