Fix --help in nested subcommands with default subcommands

[stackotter]

This fixes the issue discussed in #865. I updated the implementation of checkForBuiltInFlags's requireSoloArgument parameter to explicitly check for a single input argument rather than a single parsed argument, as some input arguments get parsed into multiple synthetic arguments, for example -help -> [-help, -h, -e, -l, -p]. #612 was trying to do this but did it in a bit of a hacky way that introduced #865. I have added a unit test that fails without these changes and succeeds with them.

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