go/build/constraint: add parser for build tag constraint expressions

This package implements a parser for the new //go:build constraint lines.
The parser also handles // +build lines, to be able to process legacy files.

This will not be used in the standard library until Go 1.17,
but it seems worth publishing in Go 1.16 so that code that
needs to process both kinds of lines once Go 1.17 comes out
will be able to build using Go 1.16 as well.

For #41184. Design in https://golang.org/design/draft-gobuild.

Change-Id: I756c0de4081c5039e8b7397200e5274f223ab111
Reviewed-on: https://go-review.googlesource.com/c/go/+/240604


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