modload: provide a clearer error for standard library packages from newer releases

modload: provide a clearer error for standard library packages from newer releases

An older version of go compiling a main module that references a
standard library package from a newer release (e.g. net/netip added in
go 1.18) currently produces a confusing error message. This changes adds
a new error message including go version diagnostics.

Fixes #48966

Change-Id: I1e8319dafcf1f67d1b1ca869fe84190c3b3f3c3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/432075
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>