Draft refactoring plan for modularization

What does this MR do and why?

Related to Create Working Group to identify the bounded co... (#423276 - closed)

These changes are extracted from #423276 (comment 1556887783).

The goal of this MR is to outline a possible iteration plan for refactoring the monolith into Packwerk packages. It does not specify a lot of details yet since some of these will be clarified by the Working Group. However, it describes expectations along each phase.

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