Disallow importing reusable ECS fields at the root level when not allowed

[jsoriano]

Elastic Package is currently allowing to import reusable ECS fields at the root level when they are not allowed. This happens for example with geo fields, that shouldn't be at the root level, but nested under some specific groups, see https://www.elastic.co/guide/en/ecs/8.9/ecs-geo.html#_field_reuse_10

Conditionally remove the logic for this, so packages using format version v3 or greater cannot import these fields this way. It should still be supported for older packages to avoid breaking changes.
Packages that need to use these fields at the root level will need to copy the resolved definition.