When canceling any package install you have a choice to "Continue" with the install or "Force Quit" Unity. If you choose to restart the next time the project is launched it will contain the canceled package.

Steps to reproduce
- Open Unity 2018.3.0b9
- Open package manager and install any package
- During install press cancel
- Notice issue a pop up will appear with a choice of "Continue" or "Force Quit".
- When restarting the project you'll notice that it will install the package anyway.

Expected:
Unity needs to uninstall packages with grace not "please kill me" and reinstall automatically on restart.

Tested: Unity 2018.3.0b12, 2018.2.1f, 2018.1.9f2
Package Manager UI Version: 2.0.3, 1.9.1,1.8.8

Not a regression as it seems this has always been the functionality.

Note. Pressing the close window button (x) will force quite Unity.