Support umbrella apps in mix package manager

[tomekowal]

Tell us more.

Hi! Thanks for your project. It really helps keeping up with our dependency upgrades :)

In Elixir, umbrella applications are sets of apps that live together in a repository and share dependencies.
E.g.

mix.lock
apps/my_app_1/mix.exs
apps/my_app_2/mix.exs

This causes problems during updates:
E.g. suppose that app_1 depends on dep A which transitively depends on T in version 1 and app_1 depends on dep B which also depends on T in version 1.

The lock then specifies:

A in version 1
B in version 1
T in version 1

Now, there is a new version of T (version 2) and A didn't yet caught up, but B also has a new version (version 2).

Let's now say that Renovate wants to update B

What happens now is that Renovate runs in my_app_2

cd apps/my_app_1
mix deps.update B

This updates B to version 2 and T to version 2, but it should not do that. If it run mix deps.update B in the root of the repo, it would fail to resolve because A still does not allow T in version 2.

I don't believe Renovate has to know that much about umbrellas and how Elixir works internally. I think that the correct solution would be to keep updating mix.exs the same way as now, but then run the mix deps.update package in places where mix.lock resides (not mix.exs).

If you agree, I am happy to make an issue and contribute the solution.