fix(products): Improve API usability.

[kpfleming]

• Add 'EnableOutput' type aliases in each product, so that users of the API don't need to know whether that product has its own EnableOutput or not. Add 'NewEnableOutput' function so that users of the API can easily construct mock output instances.

• Add 'ProductName' constant in each product, so that users of the API will have access to the canonical name of the product along with its product_id.

• Change package names to conform to Go guidelines (remove underscores).

• Change APIs to accept and return by-value instead of by-pointer, as using by-pointer structures can be difficult in generic parameter contexts and the structures involved are small (so there is no real benefit to avoiding by-value passing).

[cee-dub]

I really like the package restructuring and the consts for product names and IDs. This clarifies things and makes it easier to use this correctly.

I'm not (presently, maybe I'll change my mind?) a fan of the type aliases. It makes it seem like there are a bunch of different/new types when they're actually almost all the same productcore.EnableOutput.

[kpfleming]

The motivation for that is when a consumer of bot_management.Enable needs to know the type of the output (they won't always, but for testing they likely do), they would need to know whether that type is products.EnableOutput or bot_management.EnableOutput, and if that changed in the future it would be a breaking change. Aliasing to bot_management.EnableOutput addresses that problem well.

[kpfleming]

The ProductOutput and related types have moved back to the fastly/products package, as they are going to be useful in the CLI as a consumer of these APIs.