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?