Codify implicit startup module dependency

Previously, the implicit sort ordering of modules ensured that the "plugins" startup module ran before the "startup" startup module. That no longer holds when experimenting with alternate stores that might use a subtly different collation order (eg $:/core/modules/startup.js vs. $:/core/modules/startup/commands.js), so it's more robust to explicitly specify the dependency

core/modules/startup/plugins.js

@@ -15,6 +15,7 @@ Startup logic concerned with managing plugins
 // Export name and synchronous status
 exports.name = "plugins";
 exports.after = ["load-modules"];
+exports.before = ["startup"];
 exports.synchronous = true;
 
 var TITLE_REQUIRE_RELOAD_DUE_TO_PLUGIN_CHANGE = "$:/status/RequireReloadDueToPluginChange";