cmd/go: when resolving packages, try all module paths before falling back to the next proxy

Since we're mucking with error-propagation in modload.Query* anyway,
simplify the classification logic. Ensure that “module not found”
errors are reported as such in various places, since non-“not found”
errors terminate the module search.

Fixes #31785

Change-Id: Ie3ca5f4eec10a5f2a6037ec7e1c2cf47bd37a232
Reviewed-on: https://go-review.googlesource.com/c/go/+/177958


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>