duplicates:

• SR-20 Update swift --help to explain multitool support ("swift build", etc.)

Issue Description:

Typing 'man swift' instructs the reader to type 'swift --help' in order to get "the full list of supported options". However, doing so doesn't mention anything about the subcommands, such as `package`, `build`, and `test`.

Invoking `swift <subcommand> --help` does correctly list the options for those subcommands, but a user has to already know to type them.

So this is a request to add information to `swift -help` about the subcommands that are available. Ideally this should be dynamic, so that the mechanism that looks for subcommands by finding executables named "swift<subcommand>" can also come up with a short summary string for each (no need to repeat the full help).