x/pkgsite: grouped search fails to group some modules

[jba]

On https://beta.pkg.go.dev/search?q=xxhash, github.com/cespare/xxhash/v2 and github.com/cespare/xxhash aren't grouped together as "Other major versions."

For mvdan.cc/sh, is listed under v3, but also duplicated below it.

[julieqiu]

Here's an example for a gopkg.in package:

https://beta.pkg.go.dev/search?m=package&q=yaml

In this case gopkg.in/yaml.v3 should be grouped under gopkg.in/yaml.v2, since gopkg.in/yaml.v3 does not have a tagged version.

gopkg.in/yaml.v1 is farther down the page.

[gopherbot]

Change https://golang.org/cl/345611 mentions this issue: internal/postgres: keep only one module major version in search grouping