cargo lints: clippy dumps cargo metadata errors/warnigs into stderr

I noticed this by checking syn-1.0.54 downloaded from crates.io.

When I run cargo clippy -- --cap-lints warn -Wclippy::cargo, it looks like there is output leaking from cargo-metadata into the clippy warning such as the "warning: could not read ... Downloading crates.." stuff.

   Compiling proc-macro2 v1.0.24
    Checking unicode-xid v0.2.1
   Compiling syn v1.0.54 (/home/matthias/vcs/github/rust-clippy/target/crater/crates/syn-1.0.54)
warning: package `syn` is missing `package.keywords` metadata
  |
  = note: `-W clippy::cargo-common-metadata` implied by `-W clippy::cargo`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cargo_common_metadata

    Checking quote v1.0.8
warning: could not read cargo metadata: Error during execution of `cargo metadata`:  Downloading crates ...
  Downloaded syn-test-suite v0.0.0
error: failed to verify the checksum of `syn-test-suite v0.0.0`

  |
  = note: `-W clippy::multiple-crate-versions` implied by `-W clippy::cargo`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#multiple_crate_versions

warning: 2 warnings emitted

warning: package `syn` is missing `package.keywords` metadata
  |
  = note: `-W clippy::cargo-common-metadata` implied by `-W clippy::cargo`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cargo_common_metadata

warning: could not read cargo metadata: Error during execution of `cargo metadata`:  Downloading crates ...
  Downloaded syn-test-suite v0.0.0
error: failed to verify the checksum of `syn-test-suite v0.0.0`

  |
  = note: `-W clippy::multiple-crate-versions` implied by `-W clippy::cargo`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#multiple_crate_versions

warning: 2 warnings emitted

    Finished dev [unoptimized + debuginfo] target(s) in 6.24s

clippy @ 015496579dae282a7215a6ad17fe053342d8b825

Reproduce the issue with cargo clippy -- --cap-lints warn -Wclippy::cargo against the reported syn-1.0.54 case, starting with the cargo metadata invocation and how its output reaches Clippy diagnostics. Done means metadata download and checksum messages no longer appear inside lint warnings while relevant metadata failures remain understandable.