Search needs a main handler that will aggregate concrete search handler (Content Search Handler, Location Search Handler), similar to Persistence Handler eZ\Publish\SPI\Persistence\Handler.

This will require changing Search SPI by moving Content Search Handler into eZ\Publish\SPI\Search\Content namespace (also includes moving Location Search Handler that is under it).

This is needed in order to have cleaner dependencies in Repository and its services, but also in anticipation of Cached Search Engine and new types of search.

New SPI interface needs to be implemented, but not configured in Repository and its services. Persistence Handler does not need to be updated here. This will be handled in a following sub-task.