litex_setup installs python packages as --editable, breaking VS Code

[vvuk]

The VS Code "pylance" language server cannot find modules installed as editable using the "import finder" executable pth format. Removing the --editable fixes the issue, and litex modules are found in the editor. Is there a reason they are installed editable in non-litex-dev setups? I didn't try to "compat mode" workaround.

Ref: https://github.com/microsoft/pylance-release/blob/main/TROUBLESHOOTING.md#editable-install-modules-not-found