⌂ Including a file only once

Some files of definitions should be included only once in a project, but they need to be mentioned by all of the files that require them. I defined \includeIfAbsent to include a file if and only if it has not already been included by \includeIfAbsent. It works in a different scope from \include, so definitions in the included file need to be made global with define-public-toplevel in order to be effective.