Subcommand aliases silently shadow user-defined aliases

[ordovicia]

Problem
Users can define aliases in ~/.cargo/config.
But user-defined b, r, and t aliases do not work because they are shadowed by subcommand aliases (resp. for build, run, and test).
Cargo does not emit any warnings.

Steps

1. Add the following definition to ~/.cargo/config

[alias]
b = "foo"

2. Run cargo b
3. Cargo executes build subcommand, not foo, without any warnings

Notes

• Current Cargo warns user-defined aliases shadowed by (non-alias) subcommands here:
  

  cargo/src/bin/cargo/cli.rs

  Lines 121 to 126 in efb7972

  	(Some(_), Some(_)) => {
  	config.shell().warn(format!(
  	"alias `{}` is ignored, because it is shadowed by a built in command",
  	cmd
  	))?;
  	}

• I sent a PR that adds c alias for check (Add c alias for check #6218), which will shadow user-defined c aliases.
• Documenting subcommand aliases would tell people not to define such aliases (Aliases entirely undocumented #4391).

Output of cargo version:
cargo 1.29.0 (524a578d7 2018-08-05)