Simplify filesystem-manipulating code in wrapper

[gasinvein]

shutil.copytree() from the standard library has everything we need, namely the ability to use custom path exclude logic, so we don't really need to reinvent the wheel.

[flathubbot]

Started test build 3005

[nanonyme]

Have you locally tested it? You can first bypass helper to create incorrect data to force migration

[flathubbot]

Build 3005 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/115442/com.valvesoftware.Steam.flatpakref

[gasinvein]

Have you locally tested it?

Yeah, superficially. The idea is to trust shutil.copytree to do the right thing. The only custom code now is the one filtering ignored subpaths.

You can first bypass helper to create incorrect data to force migration

That's what I did.