cmd/go/internal/modfetch/codehost: treat nonexistent repositories as “not found”

If a go-import directive refers to a nonexistent repository, today we
treat that as an error fetching a module that actually exists.
That makes the HTTP server responsible for determining which
repositories do or do not exist, which may in general depend on
the user's separately-stored credentials, and imposes significant
complexity on such a server, which can otherwise be very simple.

Instead, check the repository URL and/or error message to try to
determine whether the repository exists at all. If the repo does not
exist, treat its absence as a “not found” error — as if the server had
not returned it in the first place.

Updates #34094

Change-Id: I142619ff43b96d0de428cdd0b01cca828c9ba234
Reviewed-on: https://go-review.googlesource.com/c/go/+/194561
Reviewed-by: Jay Conrod <jayconrod@google.com>

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