Collect warnings for path dependencies into one message printed at the end

Start by tracing Cargo's handling of rustc diagnostics produced with --error-format=json and --json=diagnostic-rendered-ansi, focusing on path dependencies and how compiler output is presented. The work is done when warnings from those dependencies are collected and a single summary such as "warning: 43 warnings emitted for all path dependencies" is printed at the end.

Describe the problem you are trying to solve

In workspaces, especially very large workspaces, warnings are scattered around the output:

warning: publicly re-exporting an item from a different crate
  --> compiler/rustc_trait_selection/src/traits/mod.rs:73:1
   |
73 | pub use rustc_infer::traits::*;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: facade crates are discouraged; import from the original crate instead

warning: 42 warnings emitted

    Checking rustc_lint v0.0.0 (/home/joshua/rustc2/compiler/rustc_lint)
    Checking rustc_typeck v0.0.0 (/home/joshua/rustc2/compiler/rustc_typeck)
    Checking rustc_traits v0.0.0 (/home/joshua/rustc2/compiler/rustc_traits)
    Checking rustc_ty v0.0.0 (/home/joshua/rustc2/compiler/rustc_ty)
    Checking rustc_mir_build v0.0.0 (/home/joshua/rustc2/compiler/rustc_mir_build)
    Checking rustc_mir v0.0.0 (/home/joshua/rustc2/compiler/rustc_mir)
    Checking rustc_passes v0.0.0 (/home/joshua/rustc2/compiler/rustc_passes)
warning: unused import: `rustc_hir as hir`
 --> compiler/rustc_mir_build/src/build/expr/into.rs:9:5
  |
9 | use rustc_hir as hir;
  |     ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

This makes it hard to see all the warnings; if there are enough crates being built they may disappear off the screen entirely and you might not see them at all.

Describe the solution you'd like

Cargo should print a sum of all the warnings at once: warning: 43 warnings emitted for all path dependencies. Since it uses --error-format=json --json=diagnostic-rendered-ansi this should be ~fairly easy to calculate from the rustc diagnostics, but don't quote me on that ;)

Ideally this wouldn't print warnings per-crate, just per-compile. However I'd rather have too much output than too little.

Notes

@joshtriplett suggested something like this in https://github.com/rust-lang/rust/pull/77492#issuecomment-703176656, so I have a strong motivation to work on it myself if someone can write up mentoring instructions :)