Auto merge of #5301 - mshal:build-plan, r=matklad

Add --build-plan for 'cargo build'

With 'cargo build --build-plan', cargo does not actually run any
commands, but instead prints out what it would have done in the form of
a JSON data structure.

Fixes #3815

diff --cc src/cargo/core/compiler/job_queue.rs
Simple merge

diff --cc src/cargo/core/compiler/mod.rs
Simple merge