Move src and built package files under the same package

[romaricpascal]

What

Move the src and built package files under a same package, allowing to share dependencies in a common package.json.
Keep shipping only the built files, excluding the source files from the package distributed on npm (either via .npmignore or the "files" field in package.json).

This change will affect longstanding pieces of work, like Exit this page or Tasklist, so we'll need to make sure to help these branches get rebased.

Why

Details for this decision can be found in our architecture repository

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

Tasks

Beta Give feedback

1. src and the built package files have been relocated under a same package (in Update Review app to import govuk-frontend via local package #3491)
2. Any wrapper package.json has been removed (in Remove built govuk-frontend from committed files #3498)
3. Outstanding branches of work have had their sources rebased to the new location of src
4. We've added pointers to the new location or src (for ex. an src/README.md file or via a symlink)