Sobelow support for nested Elixir projects

What does this MR do and why?

Describe in detail what your merge request does and why.

Nested mix.exs files are not being detected, so nested Elixir projects are not causing sobelow scans to kick off.

I set up an Elixir project to demonstrate the problem and that this MR works.

Here's a pipeline with the project not being detected, so sobelow does not run: https://gitlab.com/rossfuhrman/phoenix-nest/-/pipelines/727155484

And here I overrode the rules with the deep search from this MR and sobelow is running: https://gitlab.com/rossfuhrman/phoenix-nest/-/pipelines/727156447

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