Looks like "Searching by symbol" supports an option to limit search with package path (so, module path too). However, standard library doesn't have a common package path name so we cannot utilize this option for stdlib. And, even when std or stdlib (pseudo module names used in vuln db or in the go project) is usable, they are too common. So, as @henderjon suggested we may need a special case or search syntax.

In the mean time, users may be able to search from the source repo (e.g. https://cs.opensource.google/search?q=NewReader&sq=&ss=go%2Fgo) or source code search services like sourcegraph, etc.