Resolve assumptions in the "bundle" dependency plugin

Currently the bundle dependency plugin assumes that the bundle the document is associated with is stored in a facet field explicitly named "bundle". This is problematic when the field is not named "bundle" as was reported in the Search API integration issue #1215526: Add support for the "Bundle" dependency plugin. The assumption also caused problematic when the index contains data from multiple entities, as the plugin has no way of knowing which entity the bundle is associated with.