Hooks: All actions should be required if any are set

[charlespierce]

Currently we allow users to specify hooks for any or all of the actions associated with fetching a tool:

• distro hooks determine the download URL for a specific version of a tool
• index hooks determine the URL for the index of available versions
• latest hooks determine the URL for getting the latest version of a tool

Since these are interconnected, we should require that all of them be set if any of them are. Specifically, for users with an internal corporate repository that has a limited set of available versions of e.g. node, if they set the distro hook but not the index hook, then some developers will wind up using the public index to try to download a version of node that isn't actually available. This leads to confusing error messages and a poor developer experience.

One possible variation is that we could make the latest hook optional (or remove it entirely), since the behavior of that hook is different between Node and Yarn.