With this patch:
- `npx playwright install-deps` will install deps on Ubuntu 21.04
- launch doctor knows the package<->lib mapping for Ubuntu 21.04
**NOTE:** our webkit build, however, doesn't work on Ubuntu 21.04. Since
this is not LTS release, we're doing best-effort here.
Fixes#6405
This patch:
- refactors script to output per-browser package dependencies. This is to aid with
per-browser docker setup and Playwright github action.
- sorts package maps for both Ubuntu 18.04 and Ubuntu 20.04 alphabetically (and removes a stray dependency)
References #2926
