[BUG] Sub-entities are ignored

[grtjn]

Describe the bug
If you have an entity model with structured properties, and the models for those sub-structures are contained inside the model itself, they are omitted from the Entity dropdown in the Pipes UI.

To Reproduce

1. mlLoadModules this entity model:

{
	"info" : {
		"title" : "Test",
		"version" : "0.0.1",
		"baseUri" : "http://test.nl/datahub/entity",
		"description" : "Test case for Pipes bug report"
	},
	"definitions" : {
		"Test" : {
			"properties" : {
				"Prop1" : {
					"datatype" : "string"
				},
				"Nesting" : {
					"$ref": "#/definitions/NestingType"
				}
			}
		},
		"NestingType" : {
			"properties" : {
				"Prop2" : {
					"datatype" : "string"
				}
			}
		}
	}
}

2. reload Pipes 2.0.5 ui
3. look at Entity creation screen and check drop down. It lists Test, but not NestedType.

Expected behavior
Both Test and other embedded sub-types should all show up in drop down.

The values shows in the dropdown are influence by the backend service, which in turn runs the sparql code at this location:
https://github.com/marklogic-community/pipes/blob/master/ml-backend/src/main/ml-modules/services/vppBackendServices.sjs#L276

I suspect this code is meant to exclude external refs, but is effectively suppressing embedded types as well. I would recommend dropping the filter not exists clause entirely, and let the distinct dedupe where needed. This doesn't seem to cause any other side-effects when testing this locally.

Workaround

• Put every type in separate file, but that creates a lot of clutter, and may have other undesireable sideeffects.
• Use reverse source blocks, but that means manual work, which is undesireable too

[grtjn]

Seems related to #291, which was closed without fixes..