This patch:
- drops dependency on playwright-core in all our packages. Instead of
the dependency, packages are now built with `//packages/build_package.sh`
script.
- unifies `browsers.json` - now there's a single `//browsers.json` file
that is used to manage browser revisions.
This patch temporary switches canary publishing to `--dryn-run` from CI/CD so that we
can verify that it does sane things.
We'll unify all our package management scripts under `//packages/` in a
follow-up.
Fixes#2268
