Fixes #976 ## Background On Ubuntu Plucky, `/etc/apt/sources.list` may be absent because the system uses deb822-based configuration exclusively. When `python-apt` initializes its cache (`apt.cache.Cache(rootdir="/")`), it attempts to ensure certain files exist, including `/etc/apt/sources.list`. If the file is missing, it attempts to create it. When running as an unprivileged user (e.g. during integration tests), this results in: PermissionError: [Errno 13] Permission denied: '//etc/apt/sources.list' This failure happens in the installed-package check path, before any actual installation occurs. ## What this change does For the host `install_packages()` path only: - Replace the use of `python-apt` for: - checking whether packages are already installed - constructing the returned `package=version` list - Use `dpkg-query` instead for installed-state detection. Actual installation still uses: apt-get install ... Stage package handling and other apt-cache–based functionality remain unchanged. ## Rationale For determining whether a package is installed, `dpkg-query` is: - sufficient - does not require root - independent of apt source configuration - unaffected by deb822 migration Using `python-apt` for this specific optimization is unnecessary and causes failures on systems without `/etc/apt/sources.list`. ## Trade-offs - `dpkg-query` does not resolve virtual packages in the same way as `python-apt`. - However: - `apt-get install` still performs dependency and virtual package resolution. - The returned `package=version` list is already documented as “best effort”. - Typical build-packages (cmake, gcc, pkg-config, etc.) are concrete packages. ## Scope This change only affects the installed-check optimization and version reporting for host installs. All apt-cache usage for stage packages remains intact.