Collection

[FuhuXia]

For GSA/data.gov#4969

• By default collection children datasets are not listed. add include_collection to include them in search result, on web ui search or pacakge_search api call.
• use solr query to get_collection_package.
• add count_collection_package to tell if a dataset has children, therefore it is a collection parent

TODO

add tests

[jbrown-xentity]

LGTM!