This PR moves the puppeteer source code into separate mono-repo packages:
- `puppeteer` and `puppeteer-core` are now separated into their own
packages.
- `puppeteer-core` has a new exports called `puppeteer-core/internal`
for internal usage.
Tests and various tools have been updated to accommodate the migration.
This PR starts the monorepo migrations as per
https://github.com/puppeteer/puppeteer/issues/8922. To scope migrations,
we are only moving the `testserver` into a separate package. Further
migrations will come later.
