* Drop ensure_builtin_repositories
With the new Repository classes we have the is_builtin property, so we
can easily make sure that built-ins are not removed. This allows us to
further cleanup the code by removing the ensure_builtin_repositories
function and the ALL_BUILTIN_REPOSITORIES constant.
* Make sure we add built-ins on load
* Reuse default set and avoid unnecessary copy
Reuse default set and avoid unnecessary copying during validation if
the default is not being used.
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