Packages custom nodes should load only once

#### Purpose

Custom nodes which are belongs to packages are loaded twice. This PR makes that custom nodes to load once.

#### Modifications

During package load process path to custom nodes is not added to collection `SearchPath`. This is one of ways to fix it. Does it correct one?

#### Additional references

[MAGN-5413](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5413).
[MAGN-5414](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5414).

#### Reviewers

@pboyer, please, take a look.

Peter, we have a collection `SearchPath` with type `ObservableCollection<>`. I didn't find using of it in XAML. I propose change it type to `List<>`. What do you think?
```
public DynamoModel(){
...
this.Loader.PackageLoader.LoadPackagesIntoDynamo(preferences, EngineController.LibraryServices);
...
}
```
This loads the custom nodes from all of the packages.  In addition, it adds their dyf directories to search.  You've changed this so it is no longer the case.  

```
internal void PostUIActivation(object parameter)
{
Loader.LoadCustomNodes();
...
}
```
When PostUIActivation is called, the rest of the custom nodes are loaded by scanning the folders present in the search path (which includes package dyf folder).  

You've changed it so that the search path is largely empty, which fixes the duplicate load problem.   As the SearchPath is used rather minimally today, I think this fix is fine. 

However, this makes me wonder, why is `Loader.LoadCustomNodes` in this `PostUIActivation` call?  This mean that no custom are loaded when Dynamo starts up in headless mode EXCEPT for nodes from packages.  I have created a separate task to fix this (MAGN-5430).