cmd/doc: display a proper error when no packages are found

Fixes #32044

Change-Id: Ia220dbbe4d6851befe28dd2d4b8a1a7bc2757460
Reviewed-on: https://go-review.googlesource.com/c/go/+/178878
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/doc/pkg.go

@@ -129,7 +129,10 @@ func parsePackage(writer io.Writer, pkg *build.Package, userPath string) *Packag
 		log.Fatal(err)
 	}
 	// Make sure they are all in one package.
-	if len(pkgs) != 1 {
+	if len(pkgs) == 0 {
+		log.Fatalf("no source-code package in directory %s", pkg.Dir)
+	}
+	if len(pkgs) > 1 {
 		log.Fatalf("multiple packages in directory %s", pkg.Dir)
 	}
 	astPkg := pkgs[pkg.Name]