fix: Include rules from all apps in umbrella project

[nathanalderson]

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This change builds on #33 by including usage rules for the direct dependencies of all apps in an umbrella project. Previously, only dependencies of the root umbrella project would have their rules included.

[nathanalderson]

This probably needs a test, but I haven't taken the time yet to figure out the testing utilities. I can verify that this works on my large umbrella project. Prior to the change, mix usage_rules.sync --list (and, naturally, any generated files) would show only the usage_rules usage rules. It now also includes phoenix, which is used by one of my subprojects:

» mix usage_rules.sync --list

Notices: 

* usage_rules
    usage_rules:otp - has sub-rule
    usage_rules:elixir - has sub-rule
    usage_rules - has usage rules
* phoenix
    phoenix:phoenix - has sub-rule
    phoenix:liveview - has sub-rule
    phoenix:html - has sub-rule
    phoenix:elixir - has sub-rule
    phoenix:ecto - has sub-rule

[zachdaniel]

🚀 Thank you for your contribution! 🚀