x/pkgsite: surface VCS clone path from the go-import meta tag #46243
Description
Take golang.org/x/mod, for example:
$ curl https://golang.org/x/mod
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<meta name="go-import" content="golang.org/x/mod git https://go.googlesource.com/mod">
<meta name="go-source" content="golang.org/x/mod https://github.com/golang/mod/ https://github.com/golang/mod/tree/master{/dir} https://github.com/golang/mod/blob/master{/dir}/{file}#L{line}">
<meta http-equiv="refresh" content="0; url=https://pkg.go.dev/golang.org/x/mod">
</head>
<body>
<a href="https://pkg.go.dev/golang.org/x/mod">Redirecting to documentation...</a>
</body>
</html>
If I go to https://pkg.go.dev/golang.org/x/mod, the only link I see is https://github.com/golang/mod, which is fine for browsing the source - but not for git clone, as github is just a mirror. I should be using https://go.googlesource.com/mod for that, but pkgsite doesn't make that evident.
I get that perhaps this is an internal detail that most people shouldn't need to be aware of, but it's still relevant for peoplel wishing to contribute to a module. Especially for the x repos hosted by Gerrit - I seem to recall that you have to clone from the right URL for git-codereview to work properly.