Clarify what the contents of variables in compiler.rb are

[rileykarson]

The compiler frontend (compiler.rb) is kind of confusing to read - let's make it more clear what each variable is with clearer names, and let's pull some common subexpressions into variables.

Notable changes:

• provider and product mirror each other more closely - each of them an inputted as a _name, and each name is turned into a path to a yaml file (_yaml_path). I'd love a better name here, but calling it a config seems like a loaded term.
• provider no longer shadows provider

---

[all]

No-op

[terraform]

[puppet]

[puppet-bigquery]

[puppet-compute]

[puppet-container]

[puppet-dns]

[puppet-logging]

[puppet-pubsub]

[puppet-resourcemanager]

[puppet-sql]

[puppet-storage]

[chef]

[chef-compute]

[chef-container]

[chef-dns]

[chef-logging]

[chef-spanner]

[chef-sql]

[chef-storage]

[ansible]

[modular-magician]

I am a robot that works on MagicModules PRs!
I checked the downstream repositories (see README.md for which ones I can write to), and none of them seem to have any changes.

Once this PR is approved, you can feel free to merge it without taking any further steps.