New Subcommands

Cargo is a single binary composed of a set of clap subcommands. All subcommands live in src/bin/cargo/commands directory. src/bin/cargo/main.rs is the entry point.

Each subcommand, such as src/bin/cargo/commands/build.rs, usually performs the following:

1. Parse the CLI flags. See the command_prelude module for some helpers to make this easier.
2. Load the config files.
3. Discover and load the workspace.
4. Calls the actual implementation of the subcommand which resides in src/cargo/ops.

If the subcommand is not found in the built-in list, then Cargo will automatically search for a subcommand named cargo-{NAME} in the users PATH to execute the subcommand.