Standard help messages are not localised

ArgumentParser version: `0.2.0`
Apple Swift version 5.3 (swiftlang-1200.0.16.13 clang-1200.0.22.25)
Target: x86_64-apple-darwin19.6.0

### Checklist
- [x] If possible, I've reproduced the issue using the `master` branch of this package
- [x] I've searched for [existing GitHub issues](https://github.com/apple/swift-argument-parser/issues)

Since we now have localised resources for Swift packages, standard help messages should be localised as well. Having both localised and English text within the same environment really gives an unprofessional look.

### Steps to Reproduce
Create a basic command-line utility with commands and help messages.

### Expected behavior
Text like `OVERVIEW`, `USAGE`, `SUBCOMMANDS`, `Show the version.` should be localised.

### Actual behavior
Text mentioned above are in English, even when the rest of the strings are localised.

Start by locating the standard help-message definitions and the package's existing localized resources. Compare the handling of OVERVIEW, USAGE, SUBCOMMANDS, and “Show the version.”; the work is done when these standard messages use localization consistently with the other strings.

Written by the indexing model from the issue text.