go/build: prefer //go:build over // +build lines

Part of //go:build change (#41184).
See https://golang.org/design/draft-gobuild

- Reject files with multiple //go:build lines.
- If a file has both //go:build and // +build lines, only use the //go:build line.
- Otherwise fall back to // +build lines
- Use go/build/constraint for parsing both //go:build and // +build lines.

For Go 1.17.

Change-Id: I32e2404d8ce266230f767718dc7cc24e77b425e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/240607


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>