clean up Generator top-level and use ArgumentParser

[mayoff]

This PR makes several changes to the Generator command.

The Generator command now uses the ArgumentParser module and an AsyncParsableCommand conformance to parse the command line and run async code. This cleans up command line parsing, improves the help message, and makes it much easier to add more command line options in the future. It also eliminates the hack of using a DispatchSemaphore to synchronously wait for a Task to finish.

This PR introduces a new GeneratorCommand type which conforms to AsyncParsableCommand. The @main annotation makes GeneratorCommand the entry point for the program. I renamed main.swift to Generator.swift because we can't have a main.swift if we use the @main annotation.

This PR also introduces a new Generator type, which holds every formerly-global variable that was declared in main.swift. All of these variables are now constant (let) instead of mutable (var).

I moved many formerly global functions into extensions of Generator so they can continue accessing these variables easily.

I changed many of the helper functions to private. I found some functions that were defined in one file but only used in a different file, and moved their definitions into the file where they are used (and made them private or fileprivate).

I removed a few completely unused functions and variables.

This PR eliminates the fatal compile-time errors that prevented Generator from compiling at all with strict concurrency checking enabled. There are still several concurrency warnings, but it builds and runs successfully.

I made the changes as a series of simple, somewhat trivial commits. The Generator program builds and produces unchanged output after each individual commit in this PR. I'm happy to squash the commits if you prefer, but I thought it would be easier to review as separate commits.

[samdeane]

Whilst you're at it, can I suggest moving the Generator source code into Sources/Generator? The Xcode project could move into Sources/Generator too, for now, and be fixed up so that it can find its source.

See also #620.