x/pkgsite: count importers by module

[jba]

Change the "imported by" metric to count the number of modules whose packages import the given package, rather than the number of importing packages.

Some packages are imported by many other packages, but those importing packages are part of large modules. For example, k8s.io/apimachinery/pkg/runtime is one of the top packages in the ecosystem by importer count, but it is just a helper module for Kubernetes, whose modules contain many packages that import it.

[gopherbot]

Change https://go.dev/cl/796521 mentions this issue: DB migration for module imported count

[gopherbot]

Change https://go.dev/cl/796720 mentions this issue: internal/postgres: compute module importer counts