Fix unrecognized -help flag with default command

[natecook1000]

When there's a default subcommand that captures pass-through args, only a standalone help flag should trigger help (since the flag should otherwise go to the subcommand). This fix makes that true for single- dash help flag names, which were previously being skipped, due to single-dash flags being lexed as both one whole flag and as a group of individual short flags.

Re: swiftlang/swift-package-manager#7218
Re: rdar://120422808

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test