x/example: replace references to x/tools/go/loader with x/tools/go/packages

[adonovan]

The go/types tutorial at https://github.com/golang/example/tree/master/gotypes uses the old loader package in many examples. That package cannot handle modules correctly and is deprecated; all users should use go/packages instead. Let's update the doc to avoid steering people in the wrong direction.

See also #35872

[gopherbot]

Change https://go.dev/cl/534695 mentions this issue: gotypes: stop directing people to golang.org/x/tools/go/loader