Rename 'Plugin' & 'Config' to 'Hook' and 'HookConfig'

[charlespierce]

Implements the changes discussed in volta-cli/rfcs#25

Changes

• Combined config and plugin modules into a single modules named hook.
• Updated Inventory::fetch and Distro construction to reference the hooks and use them to resolve the URLs necessary for locating the latest version, index and specific distro download.
• Added tests to confirm the behavior of the different hook variants (Prefix and Template).

Notes

• The node hooks for latest and index are currently treated the same way, they download the index, which is then searched to find the appropriate version. This will probably need to be documented, or if Node has an URL we can hit in the same way as the Yarn latest-version endpoint to determine the latest version, we can migrate to that.
• This will likely have conflicts with Tool specs and fetch/install/pin refactoring #234, so once that is merged I will update to resolve conflicts.
• There is a slight issue with this and the Node Index Caching: The cache is just a single file, so there is no way currently for it to distinguish between sources. That means if the index is cached and the hook changes so that it should be downloading from somewhere else, the cache will still be used. Filed Make Cache of Node Index URL Aware #242 to address a more robust cache.

[charlespierce]

Resolved Merge Conflicts

[dherman]

This looks great. Just a few small suggestions.

[dherman]

Exciting work!