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

[jyn514]

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 rust-lang/rust#77492 (comment), so I have a strong motivation to work on it myself if someone can write up mentoring instructions :)