move default modules from /modules/default to /defaultmodules (#4019)

Since the project's inception, I've missed a clear separation between
default and third-party modules.

This increases complexity within the project (exclude `modules`, but not
`modules/default`), but the mixed use is particularly problematic in
Docker setups.

Therefore, with this pull request, I'm moving the default modules to a
different directory.

~~I've chosen `default/modules`, but I'm not bothered about it;
`defaultmodules` or something similar would work just as well.~~

Changed to `defaultmodules`.

Let me know if there's a majority in favor of this change.

tests/unit/modules/default/utils_spec.js

@@ -1,5 +1,7 @@
 global.moment = require("moment-timezone");
-const { performWebRequest, formatTime } = require("../../../../modules/default/utils");
+const defaults = require("../../../../js/defaults");
+
+const { performWebRequest, formatTime } = require(`../../../../${defaults.defaultModulesDir}/utils`);
 
 describe("Default modules utils tests", () => {
 	describe("performWebRequest", () => {