Stop removing underscores from CodingKey names in InputKey

[natecook1000]

When a property wrapper is applied to a property, the property's storage is given a name with a prefixed underscore. That is, for a property named x, the actual storage is named _x.

That prefixed storage is what is visible through reflection, so when building an ArgumentSet from a command type's mirror, we need to remove the leading underscore. This is done when creating an InputKey for each property.

However, InputKeys are also created from CodingKeys during decoding of a command. These CodingKeys do not have the leading underscore that is added by the property wrapper, so any underscores that appear are actually from the declaration of the property with an underscored name. Removing leading underscores from CodingKey names results in a mismatch when trying to find the decoded value. For example, for a simple command declared like this:

@main
struct Repro: ParsableCommand {
    @Option(name: .customLong("arg"))
    var _arg: String?

    func run() throws {
        print(self._arg ?? "nil")
    }
}

Running: repro --arg foo fails to parse with the error:

Error: Unknown option '--arg'. Did you mean '--arg'?
Usage: repro [--arg <arg>]
  See 'repro --help' for more information.

This change simplifies the InputKey type to use an array path instead of an indirect enum and removes the leading underscore dropping when creating an InputKey from a CodingKey.

rdar://104928743

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