PR from Candidate 4C1BB6

[jlengstorf]

Anonymized!

Original PR text:

feat(CLI): add a flag for Yarn Plug'n'Play

This pull request is separated in multiple steps

1. add the new flag and propagate it
2. add the pnp webpback plugin
3. fix use of transitive dependencies (not allowed in pnp mode) (luckily these are all inside gatsby's monorepo)
4. fix "main" in a dependency: facebook/relay#2573 & yarnpkg/yarn#6682 (after I talked to yarn maintainers)
5. don't use transitive dependencies at "runtime" (i.e. in cache and public folder of a Gatsby app)

The remaining step (5) is not implemented in this PR (because they touch too many internals of Gatsby):

• ./.cache
• ./public
• eslint

These three steps rely heavily on transitive dependencies. For the first two you can fix it likely by making @reach/router, lodash, prop-types, core-js either not used until someone installs it, or using something like require.resolve. For eslint I couldn't dig deeper which part of Gatsby is relevant, and where the modules eslint-plugin-import, eslint-plugin-flowtype should be listed as direct dependencies.