cmd/go: parallelize package loading

load.PackageAndErrors now preloads data used to build load.Package
structures. Multiple packages may be preloaded in parallel, so this
parallelizes most of the package loading work.

The actual package construction and error-checking process is still
sequential, since this process needs to detect and report cycles.

Fixes #29758

Change-Id: Icf37e6669836ce8aad076e34fd895f97f4f3f9e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/161397


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