cmd/go: fix //go:binary-only-package check

The use of a prefix check was too liberal.
Noted in review after submit.

Change-Id: I4fe1df660997efd225609e818040b8392fab79f0
Reviewed-on: https://go-review.googlesource.com/23375
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -2804,7 +2804,8 @@ func TestBinaryOnlyPackages(t *testing.T) {
 	os.Remove(tg.path("src/p1/p1.go"))
 	tg.mustNotExist(tg.path("src/p1/p1.go"))
 
-	tg.tempFile("src/p2/p2.go", `
+	tg.tempFile("src/p2/p2.go", `//go:binary-only-packages-are-not-great
+
 		package p2
 		import "p1"
 		func F() { p1.F(true) }

src/go/build/build.go

@@ -1151,7 +1151,7 @@ func (ctxt *Context) shouldBuild(content []byte, allTags map[string]bool, binary
 		}
 		line = bytes.TrimSpace(line)
 		if bytes.HasPrefix(line, slashslash) {
-			if bytes.HasPrefix(line, binaryOnlyComment) {
+			if bytes.Equal(line, binaryOnlyComment) {
 				sawBinaryOnly = true
 			}
 			line = bytes.TrimSpace(line[len(slashslash):])