Handle base fields imported from nested ECS definitions in a more generic way

[jsoriano]

More future-proof implementation of #777.

Previous implementations considers base and tracing the only groups that can
contain top-level fields. Implementation on this change checks field by field to see
if they contain base fields.

It also handles the case of having groups with both nested and top-level fields, something
that doesn't happen now, but just in case, to don't lose definitions if it happens at some
moment.

[elasticmachine]

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• Start Time: 2022-04-04T09:09:35.694+0000

• Duration: 31 min 53 sec

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