Add support for child configs

[jimmya]

The project I work in uses sourcery and consists of quite a lot packages. Having all these configs in a single .sourcery.yml file is becoming quite hard to manage (350+ lines).

This PR allows for child configs, in order to break configs into smaller sub configs and also directly allow for them to be run independently.

A config with child config would look like this:

configurations:
    - child: ./.child_config.yml
    - child: Subdirectory/.another_child_config.yml

Whereas the child config would be a regular sourcery config file (or have further child configs). The existing config keeps working as is, so you could have a regular config with also child configs in it.

[art-divin]

Thank you very much @jimmya 🤝 Once CI will pass, I will merge this one.

Next release I plan to do in 2-3 weeks or so, quite busy. There is plenty of new bug reports already.

[art-divin]

hey @jimmya ,

the following test fails on CI:

/Users/runner/work/Sourcery/Sourcery/Tests/SourceryLibTests/ConfigurationSpec.swift:109: error: -[SourceryLibTests.ConfigurationSpec Configuration__given_config_file_with_child_configurations__resolves_each_child_configuration] : failed - expected to equal </Users/runner/work/Sourcery/Sourcery/.build/x86_64-apple-macosx/debug/Sourcery_SourceryLibTests.bundle/Configs//>, got </Users/runner/work/Sourcery/Sourcery/.build/x86_64-apple-macosx/debug/Sourcery_SourceryLibTests.bundle/Configs/Sources>