Reproduction steps:
1. Create a new project and open it
2. In the top toolbar open Window/UI Toolkit/Debugger
3. Try selecting anything in the dropdown in the top left corner of the UI Toolkit Debugger window
4. Notice how the hierarchy appears on the left side of the window and the names of Visual Elements are displayed
5. Close the UI Toolkit Debugger window and open Package Manager
6. Click on "+" in the top left corner of the Package Manager and choose "Add package from git URL"
7. Type in "com.unity.ui" and click on "Add"
8. Repeat steps 2-3

Expected result: The names of Visual Elements are displayed in the UI Toolkit Debugger both when UI Toolkit is installed and when it's not
Actual result: The names of Visual Elements are displayed in the UI Toolkit Debugger only when the UI Toolkit is missing from the project

Reproducible with: 1.0.0-preview.12, 1.0.0-preview.14 (2020.3.12f1, 2021.1.11f1)
Not reproducible with: 2021.2.0a21 (UI Toolkit is integrated into the editor itself)
Couldn't test with: 2018.4, 2019.4 (UI Toolkit package is not available)

Note: Console logs out a warning when clicking Visual Elements in the UI Toolkit Debugger is opened after installing the UI Toolkit package "Font not found for path: Fonts/consola.ttf UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)"