[Lens] Rename "indexpattern" datasource

[flash1293]

Lens contains an abstraction layer for multiple data sources. So far, only a single data source is implemented, with a second one planned for the near term.

The current datasource is called indexpattern which is the old name for data views which are heavily used by the indexpattern datasource. However, based on our current plans data views are a central building block leveraged by all datasources which makes the name a very bad fit for the current setup.

Because of this, the data source should be renamed to "form_based" - for text based query languages we are later going to add a "text_based" datasource which powers esql and co.

[elasticmachine]

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