Importing module that imports another module(s) in Python

I'm currently trying to rewrite an old app, and becuase first things first, I want to clean up the structure a little bit. I encountered a problem with importing modules, and I've exhausted few options found over the web, so it's time to ask for help.

The relevant structure is:

root/
   __init__.py
   app.py
   interface/
      __init__.py
      darkMotive.py
      mainWindow.py

app.py

import interface.mainWindow

if __name__ == "__main__":
    root = tk.Tk()
    main = mainWindow(root)
    title = root.title("app")
    main.pack(side="top", fill="both", expand=True)
    root.mainloop()

mainWindow.py

import darkMotive
import tkinter as tk

class mainWindow(tk.Frame):
    def __init__(self, *args, **kwargs):
    [rest of the code]

darkMotive.py

import tkinter as tk
[other classes definitions]

My problem is that when I try to run app.py, I get this error:

Traceback (most recent call last):
  File "X:/x/xxx/app.py", line 2, in <module>
    import interface.mainWindow
  File "X:\x\xxx\interface\mainWindow.py", line 1, in <module>
    import darkMotive
ModuleNotFoundError: No module named 'darkMotive'

But when I run mainWindow.py alone, it works.
What do I do wrong? Could you point me in the right direction? Shouldn't module darkMotive be imported with mainWindow into app.py's namespace?