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.

.gitignore

@@ -54,9 +54,8 @@ Temporary Items
 .directory
 .Trash-*
 
-# Ignore all modules except the default modules.
+# Ignore all modules
 /modules/*
-!/modules/default
 
 # Ignore changes to the custom css files but keep the sample and main.
 /css/*