Add `depends_on` stanza to DSL.

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