Packaged Standard Library missing modules

# Fact

While exploring #381, trying to package the Mu Editor on Windows, I realized that the packaged Python environment is missing modules from the Standard Library.

In particular I realized that `tkinter`, `pip`, and `venv` are missing.

Then I tried the same on macOS and confirmed that they're missing too.

# Question

While I do understand that by creating and using an alternate support package these limitations can be overcome, I wonder if, from a user/packager experiece, it wouldn't be desirable for the packaged environment to be as capable as the environment in which the original code is developed and tested and, thus, have it be that way by default?

# Thought

(warning: crazy idea ahead) :)

I wonder if instead of using a pre-built support package `briefcase` could instead "copy and adjust" the current Python installation into the packaging `src` directory. Maybe some adjustments might be needed to ensure the "copied" environment is relocatable and works nicely, but how hard could that be? (grin)

Of course, for some target platforms that is not an option given that `briefcase` is packaging with a foreign target like iOS targets being packaged on macOS, for example.

Thoughts?