cmd/go: add tracing for querying and downloading from the proxy

This CL adds tracing spans for modload.queryPattern, modload.queryProxy,
modload.QueryPattern, modload.QueryPattern.queryModule,
modload.queryPrefixModules and modfetch.Download.

Updates #38714

Change-Id: I537c7fa4f466c691c1b60ec73ef8a2277af49cd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/242786


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>