go/packages: correct logging of driver stderr
go/packages: correct logging of driver stderr

This moves the GOPACKAGESPRINTDRIVERERRORS and stderr.Bytes() checks to after
the driver command is run. Before, these were checked before the driver command
was run and so stderr was always empty.

Change-Id: I8fb3d45eab5dacee3871d454038fbcc1b6351cb6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/207581
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools