For #56986, go/build needs to report up to cmd/go
about //go:debug lines found in the source code.
Rather than make a special case for //go:debug,
this change gathers all top-level directives above the
package line and includes them in the result.
The go command's module index must match go/build,
so this CL contains the code to update the index as well.
A future CL will use the //go:debug lines to prepare the default
GODEBUG settings, as well as rejecting such lines in non-main
packages.
Change-Id: I66ab8dc72f9cd65c503b10b744367caca233f8a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/453603
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
