x/pkgsite: support go-import subdir #75258
Description
As of Go 1.25, the go-import meta syntax has a new feature: https://tip.golang.org/doc/go1.25#go-command (search for subdir).
The code at https://go.googlesource.com/pkgsite/+/refs/heads/master/internal/source/meta-tags.go needs to be updated to understand it.