Remove `datacube-search` CLI by moving it's features into the `datacube` CLI

[omad]

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The decision to expose two top level commands was a bit of a strange decision in the first place.

It was supposed to be something like user features vs administrator features. But there's very useful features for users in the datacube CLI like system check and exploring all the database objects, which makes this split unnecessary, and hides some quite useful functionality.

I think the extra features in the datacube CLI could be implemented in any minor release. The datacube-search tool should be marked as deprecated first, before being removed in a major release.