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.

Fix #3883
Revert 2fb5ead3

Requirement#modify_build_environment may depend on the side effects of
Requirement#satisfied?, so make sure the latter is called at least once
for each requirement during formula installation. Prior to b70b5429,
Requirement#satisfied? would usually be called during Build#expand_reqs,
but that is longer the case.

This has been a nightmare in terms of the complexity to our dependency
system and the whack-a-mole required on bugs. If a Requirement resolves
to a Formula it should just use `depends_on "formula"` instead. This
matches the effective behaviour all users of bottles (the vast majority
of users and installs) and what we're doing in Homebrew/homebrew-core.

so that it includes the commit.

These can be useful for later inspection or upload by `gist-logs` if
there's a failed install and it's unclear from logs alone what options
were used.