[SPARK-13738][SQL] Cleanup Data Source resolution

Follow-up to #11509, that simply refactors the interface that we use when resolving a pluggable `DataSource`.
 - Multiple functions share the same set of arguments so we make this a case class, called `DataSource`.  Actual resolution is now done by calling a function on this class.
 - Instead of having multiple methods named `apply` (some of which do writing some of which do reading) we now explicitly have `resolveRelation()` and `write(mode, df)`.
 - Get rid of `Array[String]` since this is an internal API and was forcing us to awkwardly call `toArray` in a bunch of places.

Author: Michael Armbrust <michael@databricks.com>

Closes #11572 from marmbrus/dataSourceResolution.