Use Yarn workspaces & Lerna?

There was some discussion of this way back in https://github.com/prettier/prettier/issues/769, but since the plugin API was built and the code was modularized into "internal plugins", it's starting to make sense to switch to more of a monorepo approach.

Proposed packages:

Each package would produce (via Rollup) one CommonJS bundle shipped to npm.

Advantages:

• External plugins would only need to depend on @prettier/doc, which would remove the current circular dependency (@prettier/python require('prettier') which require('@prettier/python')).
• Using dependency (package.json) validation, we're able to enforce that parsers and printers are represented using the public API, which means we could easily extract them into their own repositories, and vice versa.
• People can use the @prettier/doc package as a standalone module.

Disadvantages:

• Can no longer do npm install prettier/prettier (https://github.com/npm/npm/issues/2974). Perhaps we could do an automated push to a bundled branch that is pre-bundled so that npm install prettier/prettier#bundled works? Alternatively publish every merge to npm under a next dist-tag?
• ❓

Start by reviewing the proposed mappings from src/common, src/config, src/cli, src/doc, src/language-* and src/main to packages/*, along with the package dependencies. Then inspect the Rollup bundling and package.json validation implications. Done would require an agreed monorepo structure and a migration plan that preserves the listed package and plugin relationships.