use single root package.json, do not build intermediate shared code
Created by: sqs
This branch uses a single root package.json. See https://sourcegraph.com/github.com/sourcegraph/sourcegraph@master/-/blob/doc/dev/typescript/build.md and https://sourcegraph.slack.com/archives/C07KZF47K/p1541878567271600 for context.
- dev/launch.sh and enterprise/dev/start.sh work
- unit tests work, buildkite works, etc.
- browser extension build works
- browser extension storybooks work
- vscode typescript support works
- prettier/tslint/etc. work
I am going to merge it now because (1) the plan has been reviewed/discussed already and the PR is not going to be very useful to review since it's so large/mechanical, and (2) it would otherwise start generating a lot of merge conflicts.