Formulae can now pass build options to dependencies. The following
syntax is supported:
depends_on 'foo' => 'with-bar'
depends_on 'foo' => ['with-bar', 'with-baz']
If a dependency is already installed but lacks the required build
options, an exception is raised. Eventually we may be able to just stash
the existing keg and reinstall it with the combined set of used_options
and passed options, but enabling that is left for another day.
The DependencyCollector tests are really integration tests, while the
rest are closer to real unit tests. Split them up so that the tests can
be run in isolation on a per-class basis.
