src/plugman/install.js
function copyPlugin (plugin_src_dir, plugins_dir, link, pluginInfoProvider) {
    var pluginInfo = new PluginInfo(plugin_src_dir);
    var dest = path.join(plugins_dir, pluginInfo.id);

    if (link) {
        events.emit('verbose', 'Symlinking from location "' + plugin_src_dir + '" to location "' + dest + '"');
        fs.removeSync(dest);
        fs.ensureSymlinkSync(plugin_src_dir, dest, 'junction');
    } else {
        events.emit('verbose', 'Copying from location "' + plugin_src_dir + '" to location "' + dest + '"');
        fs.copySync(plugin_src_dir, dest);
    }
    pluginInfo.dir = dest;
    pluginInfoProvider.put(pluginInfo);

    return dest;
}
function copyPlugin (plugin_src_dir, plugins_dir, link, pluginInfoProvider) {
    var pluginInfo = new PluginInfo(plugin_src_dir);
    var dest = path.join(plugins_dir, pluginInfo.id);

    if (link) {
        events.emit('verbose', 'Symlinking from location "' + plugin_src_dir + '" to location "' + dest + '"');
        fs.removeSync(dest);
        fs.ensureSymlinkSync(plugin_src_dir, dest, 'junction');
    } else {
        events.emit('verbose', 'Copying from location "' + plugin_src_dir + '" to location "' + dest + '"');
        fs.copySync(plugin_src_dir, dest);
    }
    pluginInfo.dir = dest;
    pluginInfoProvider.put(pluginInfo);

    return dest;
}
Copy or link a plugin from plugin_dir to plugins_dir/plugin_id.