x/pkgsite: migrate to v1 vulnerability database

[tatianab]

[gopherbot]

Change https://go.dev/cl/474255 mentions this issue: internal/frontend, internal/vulns: isolate references to x/vuln repo

[gopherbot]

Change https://go.dev/cl/474537 mentions this issue: internal/frontend, internal/vuln: replace getVulnEntries with vuln.Client

[gopherbot]

Change https://go.dev/cl/476555 mentions this issue: internal/vuln: add support for v1 vulndb client

[gopherbot]

Change https://go.dev/cl/477015 mentions this issue: add newSource and newClient functions

[gopherbot]

Change https://go.dev/cl/476556 mentions this issue: cmd/frontend, internal/vuln: add vulndb v1 experiment