allow multiple depends_on stanzas + refactor

also:
* disallow combining depends_on :macos comparison expressions
* ohai message when depends_on is present
* explicit accessors for :cask and :formula
* simplify some logic and flatten nested conditionals
* reflow incidental doc
DSL: add depends_on :x11
Merge pull request #7845 from rolandwalker/depends_on_x11

DSL: add `depends_on :x11`, permit multiple `depends_on` stanzas