x/pkgsite: search doesn't recognise some well-known tools

[ainar-g]

The search on the website sometimes doesn't show well-known tools as the first results. Here are some cases where it doesn't:

• https://pkg.go.dev/search?q=errcheck

• https://pkg.go.dev/search?q=gopls

• https://pkg.go.dev/search?q=nilness

• https://pkg.go.dev/search?q=unused

For comparison, here are some where it does:

• https://pkg.go.dev/search?q=staticcheck

• https://pkg.go.dev/search?q=shadow

[toothrot]

/cc @julieqiu

See also #37810

[ldemailly]

I'll add https://pkg.go.dev/search?q=fortio to the list (doesn't get to https://pkg.go.dev/fortio.org/fortio yet shows some outdated forks in the results)