go/build: add BuildTags to Context, allow !tag
This lets the client of go/build specify additional tags that
can be recognized in a // +build directive. For example,
a build for a custom environment like App Engine might
include "appengine" in the BuildTags list, so that packages
can be written with some files saying
// +build appengine (build only on app engine)
or
// +build !appengine (build only when NOT on app engine)
App Engine here is just a hypothetical context. I plan to use
this in the cmd/go sources to distinguish the bootstrap version
of cmd/go (which will not use networking) from the full version
using a custom tag. It might also be useful in App Engine.
Also, delete Build and Script, which we did not end up using for
cmd/go and which never got turned on for real in goinstall.
R=r, adg
CC=golang-dev
https://golang.org/cl/
5554079