is_in_apt_repo() gets confused in certain output

[TolisSth]

`is_in_apt_repo()` gets confused on the special output of `freecad` we should investigate and resolve.

[michadasis]

bool Downloader::is_in_apt_repo(const QString &package_name) {
    QProcess process;
    process.start("apt-cache", QStringList() << "show" << package_name);
    process.waitForFinished();

    QString stdout_output = process.readAllStandardOutput();

    // exit code 0 alone is not enough for virtual packages (e.g. freecad) also
    // exit 0 but produce no real package record. Check that stdout contains an
    // actual "Package:" field to confirm it is a real, installable package.
    if (process.exitCode() == 0 && stdout_output.contains("Package:")) {
        emit status_message("Found: " + package_name);
        QCoreApplication::processEvents();
        return true;
    } else {
        emit status_message("Not in repos: " + package_name);
        QCoreApplication::processEvents();
        return false;
    }
}