go/build: remove unnecessary copies of package variables
go/build: remove unnecessary copies of package variables

These variables never change, we don't need second copies of them.

Also rename bPlusBuild to plusBuild, since it is the only remaining
variable with a "b" prefix.

Change-Id: I97089b001d23e9b0f2096e19d9ceed7a9bbb377d
Reviewed-on: https://go-review.googlesource.com/c/go/+/429636
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Benny Siegert <05f4aee36fd25d0de2a07789fd6e73398db492cc@gmail.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go