Separate ListPackages, PackageTree into subpackage

[sdboyer]

The primary gps package is becoming somewhat unwieldy, so it's worth starting to separate out logic dealing with discrete subsystems into their own subpackages. Unfortunately, a lot of gps' systems are deeply interwoven, so this isn't easy to do in a lot of areas.

One exception, though, is the family of functions around PackageTree - they're quite well self-contained. They're also growing, and are likely to continue to grow as we refine that subsystem. So, they're a good first candidate for separation into a subpackage.

[sdboyer]

Oh, this is all done. Thanks, @narqo!

[narqo]

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