Collapse aggregation packages

[jpountz]

The fact that every aggregation has its own package requires to make too many classes public. We should look into collapsing packages and making as many classes package-private as possible. Ideally we'd do this close to the 6.0 release to keep backporting easy.

[sophiec20]

@jpountz Should this be an open blocker?

[jpountz]

Bumping the version, it's now too late to do this change. Thanks for the ping.

[polyfractal]

@elastic/es-search-aggs