add --explain subcommand

Author: llogiq

Cargo.toml

@@ -33,6 +33,7 @@ tester = "0.9"
 regex = "1.5"
 # This is used by the `collect-metadata` alias.
 filetime = "0.2"
+walkdir = "2.3"
 
 # A noop dependency that changes in the Rust repository, it's a bit of a hack.
 # See the `src/tools/rustc-workspace-hack/README.md` file in `rust-lang/rust`