cmd/go: for missing binary-only package, say where it should be

Before this CL the user effectively has to guess at the expected
location of a binary-only package. While the location is normally
obvious ($GOPATH/pkg/GOOS_GOARCH/PATH/PKG.a) it is much less so when
building with options that implicitly add an -installsufix option.

Fixes #26590

Change-Id: I753ef54d6dcf733bb456dba65a4a92e4db57a1b0
Reviewed-on: https://go-review.googlesource.com/125818
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/work/exec.go

@@ -411,7 +411,7 @@ func (b *Builder) build(a *Action) (err error) {
 		if b.IsCmdList {
 			return nil
 		}
-		return fmt.Errorf("missing or invalid binary-only package")
+		return fmt.Errorf("missing or invalid binary-only package; expected file %q", a.Package.Target)
 	}
 
 	if err := b.Mkdir(a.Objdir); err != nil {

src/cmd/go/testdata/script/binary_only.txt

@@ -0,0 +1,10 @@
+# check that error for missing binary-only says where it should be
+! go build b
+stderr pkg[\\/].*a\.a
+
+-- a/a.go --
+//go:binary-only-package
+
+package a
+-- b/b.go --
+package b; import "a"