connector: load loopback models from filesystem

Add a new internal property `ModelDefinition.readonly` to signal whether
the definition can be edited by users.

Modify connector to search `node_modules/loopback` for model
definitions.

Fix `ConfigFile` methods to correctly support subclassing.
Handle shorthand property definition

LDL supports a shorthand notation for defining model properties:

```json
{
  "properties": {
    "name": "string",
    "tags": ["string"]
    "anonymousObject": { "name": { "type": "string" } }
  }
}
```

This commit modifies the code handling embedded objects to normalize
shorthand definitions like the above.