Validate command dispatch registration in router

[slashdotdash]

When using the Commanded.Commands.Router to configure command dispatch, it would be helpful to validate that the command, handler, and aggregate modules exist.

In the following example, if the two module alias statements are forgotten then command dispatch will fail (UndefinedFunctionError module is not available).

defmodule MyApp.BankRouter do
  use Commanded.Commands.Router

  alias MyApp.OpenAccount
  alias MyApp.BankAccount

  dispatch OpenAccount, to: BankAccount, identity: :account_number
end

Instead, this should cause a compilation failure, prompting the user that the modules cannot be found.

[astery]

Definitely, stumbled on that several times.