Corrected ElementType specifying

#### Purpose

For some cases `NodeSearchElement.ElementType` is set incorrectly. This PRs fixes it.

Now we have next schema:
1. For Package Custom Node or DLL `ElementType` is set to `Package`.
2. For imported library: `CustomDll`.
3. For any other custom node (loaded from dyf, just created): `CustomNode`.

#### Modifications

During implementation I was going through load process of Dynamo and have fixed some issues with setting correct `ElementType`.

#### Additional references

[MAGN-4653](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4653).

#### Reviewers

@Benglin, please, take a look
I have thought more about the code during my trip to home. I think we can add new parameter to function `LoadCustomNodes` `elementType`. It can be used instead of hardcoded `Package`.