Add fx.Module (#830)

This adds fx.Module Option which is a first-class object for supporting scoped operations on dependencies. A Module can consist of zero or more fx.Options.

By default, Provides to a Module is provided to the entire App, but there is a room for adding an option to scope that to a Module. Module can wrap Options such asSupply/Extract, Provide, and Invoke but there are some Options that don't make sense to put under Module. For example, StartTimeout, StopTimeout, WithLogger explicitly errors out when supplied to a Module.

Implementation-wise, a Module corresponds to dig.Scope which was added in uber-go/dig#305. Extra bookkeeping is done by the module struct which contains the provides and invokes to a Scope.

Co-authored-by: Abhinav Gupta <mail@abhinavg.net>
Co-authored-by: Abhinav Gupta <abg@uber.com>
Co-authored-by: Abhinav Gupta <mail@abhinavg.net>