More complete YAML provider implementation.

[robertwb]

Transforms defined in a provider in YAML are now expanded in context, which means in particular that they can reference other transforms (including themselves) that have been exported.

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[liferoad]

Let us add at least one example later to show how to use this. thanks.

[robertwb]

Thanks for the quick review! I've got a full example ready to add to https://github.com/apache/beam-starter-python-provider as well once this is in.