If a legacy multi remote Gemfile depends transitively on a default gem,
then in standalone mode we'd fail to fetch the proper version from the
source that includes it, since we were adding it to `specs` (instead of
`remote_specs`), which was already including the default version of the
gem, and thus preventing the remote version from "overwriting that" and
being added to the index. We should add it to the `remote_specs` index
directly instead.

https://github.com/rubygems/rubygems/commit/05f4f9dfc0