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 created PR in master https://github.com/DynamoDS/Dynamo/pull/3362 which renames this method. It is under review. In that PR function is renamed.

As soon as that PR merged I will update this code. Task: [MAGN-5627](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5627).

Now, please, let's use this behavior.