Help needed with umbrella-settings

I am working on an application that has multiple parts. Therefore, it was split in an Umbrella application. However, I am having trouble figuring out what should be put in which mix.exs file. Starting the project normally works fine, but when attempting to compile the application for production-use, (using edeliver), I get strange errors.

One application (payment_dispatcher) is a client that consumes an external API.
One application (heads) is a Phoenix server.
heads needs payment_dispatcher as dependency, as the server needs access to parts of the API.

Basically:

appname
--heads
----phoenix
----payment_dispatcher
----(... some other things)
--payment_dispatcher
----httpoison
----(... some other things)

When edeliver uses the following command to compile the files:

APP="appname" MIX_ENV="prod" AUTO_VERSION="" SKIP_RELUP_MODIFICATIONS="" RELUP_MODIFICATION_MODULE="" mix do clean, deps.compile, compile

I get an error that HTTPoison.Base, one of the modules inside the :httpoison dependency of payment_dispatcher cannot be found.

I have listed :httpoison inside apps/payment_dispatcher/mix.exs’s deps and applications lists. But the error keeps happening.

I am unsure as to:

• Why above error happens. Is compilation in the wrong order? How can I change that?
• Where to put payment_dispatcher as dependency: In mix.exs or apps/heads/mix.exs? (as the Phoenix app depends on it)

Any help would be appreciated.