This introduces a set of classes for serializing/deserializing package

data stored in a repository, and classes for interacting with the

repository. It can be used to query packages, the available versions,

installation instructions, and more.
Each entry in the repository is a PackageBundle, which bundles together

a list of channels (PackageChannel). Each of these manages state for a

range of versions (1.0.x, 1.5.x, 2.0.x, etc.), and may also be mapped to

one or more aliases (beta, stable, etc.).
Each channel lists all available releases (PackageChannel), and also

contains rules (PackageRules) on how to install any given version.
These objects are populated with data from the repository through

deserialization class methods that they provide. That data comes from a

PackageDataLoader subclass, which knows how to fetch and deserialize

JSON data from a backend (the main package repository, a testing

repository, etc.).
Currently, there's no data loader implemented to talk to any

HTTPS-backed repository, nor are there any classes for

managing/installing packages beyond fetching data on them. Those will

come in future changes.

Unit tests pass.
Implemented a test command locally, and used it to introspect package data.