repos: add x/vuln and remove build for x/vulndb
repos: add x/vuln and remove build for x/vulndb

x/vuln now contains code previously in x/vulndb, and is added to repos.

The description for x/vulndb is updated.

Change-Id: I5bbecf6b97c61c36f75fb404b8040fa1cc2fc7e5
Reviewed-on: https://go-review.googlesource.com/c/build/+/360442
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build