go.tools/godoc: mention sub-repos and package indexes on packages page

Also several cosmetic tweaks to the package page.

LGTM=r, bradfitz
R=bradfitz, r
CC=golang-codereviews
https://golang.org/cl/128490043
diff --git a/godoc/server.go b/godoc/server.go
index f81a02d..900f9b7 100644
--- a/godoc/server.go
+++ b/godoc/server.go
@@ -254,8 +254,10 @@
 	// special cases for top-level package/command directories
 	switch tabtitle {
 	case "/src/pkg":
+		title = "Packages"
 		tabtitle = "Packages"
 	case "/src/cmd":
+		title = "Commands"
 		tabtitle = "Commands"
 	}