x/pkgsite: change go.mod link to proxy.golang.org's go.mod link

[hyangah]

Currently, the go.mod link in the details section is linked to the source repository's go.mod file.

Example: https://pkg.go.dev/github.com/pedroalbanese/edgetk@v1.2.3

But it's not guaranteed that the source still hosts the file or the same content.
Instead use the link to the proxy.golang.org's .mod endpoint. For example,

https://proxy.golang.org/<proxy-protocol-encoded-module-path>/@v/<canonical-version>.mod