Replace with `Homebrew.args.debug?`.

Part of #5730.

- ensure that cases where `:build` and `:test` both apply to a
  dependency that we handle it appropriately
- fix reintroduced regression
  (https://github.com/Homebrew/brew/pull/6471#issuecomment-533414923).
  Fixes #6470.

Also:
- when `brew test` or `brew postinstall` is run allow `Resource#stage`
  to fetch the resource.
- make `Formula#fetch` and `Resource#fetch` fetch external patches too.

Follow-up from #7549 and #7546.

Fixes #7008

Fixes https://github.com/Homebrew/brew/issues/5427.

Replaces our serialization of child process
errors via Marshal with JSON, preventing
unintentional or malicious code execution outside
of the build sandbox.

Additionally, adds tests for the new behavior.

Replaces our serialization of child process
errors via Marshal with JSON, preventing
unintentional or malicious code execution outside
of the build sandbox.

Additionally, adds tests for the new behavior.

Adjust the rules based on the current codebase. Remove various enable,
disables and default values that are unnecessary. Add more comments
explaining why. Make minor changes needed to enable a few more rules.

Reuse more code to avoid errors due to duplication.