DSL: add depends_on stanza

[rolandwalker]

For forward-compatibility, intentionally undocumented. Currently
works only as a synonym: depends_on :formula substitutes for
depends_on_formula, which is intended to be removed.

However, this PR also provides additional depends_on keys for
future expansion of functionality:

• :cask
• :macos
• :arch
• :x11
• :java

The symbol names were chosen to match those used in Homebrew Formulae.

References: #4688, #2001

[nanoxd]

I'm not sure if it was intended, but it's missing a test for raise CaskInvalidError.new(self.title, "'depends_on' stanza may only appear once") at least for the time being.

[rolandwalker]

I intentionally left out that test, as it represents undesired behavior. My philosophy is that tests represent the spec.

In fact, if I knew Ruby testing frameworks better, I would have put in the exact opposite test, marked as "allowed to fail", and used that to track when the existing limitation gets fixed.

It's awesome to have your close review on these patches. I used Set here, per your advice earlier, and am about to revise the previous PRs to do the same. I was pleasantly surprised to find that the Set could be splatted naturally with attr_accessor *VALID_KEYS.

[rolandwalker]

@radeksimko I stubbed depends_on :x11 here as promised in an earlier thread.