fx.Extract: Deprecate in favor of populate (uber-go#889)

fx.Extract is an older, poorer version of fx.Populate.
fx.Populate works on any value type, and fx.Extract requires building a
placeholder struct.

We meant to deprecate it right away, but because of widespread use of
staticcheck at the time, we marked it as "will be deprecated".

This finally deprecates it.

extract.go

@@ -33,8 +33,7 @@ var _typeOfIn = reflect.TypeOf(In{})
 // container on application initialization. The target MUST be a pointer to a
 // struct. Only exported fields will be filled.
 //
-// Extract will be deprecated soon: use Populate instead, which doesn't
-// require defining a container struct.
+// Deprecated: Use Populate instead.
 func Extract(target interface{}) Option {
 	v := reflect.ValueOf(target)