ConfigReader: merge multiple lists of targets together?

[jonathanmaw]

We are able to load a directory full of .yml files for Lifecycle's config.

Lifecycle supports listing multiple targets and having them all update at once from a single source.

The config format supports this, by expecting:

source:
  module: ...
  ...
targets:
  - module: ...
    ...
  - module: ...
    ...

However, because ConfigReader uses dict.update() to merge config from previous files, lists of targets overwrite each other.
This means you can't have different targets as separate config files, e.g. a directory containing source.yml, target1.yml, target2.yml
Do we want to explicitly merge these lists together?
Do we want to drop support for using multiple targets at once?