Module will not install due to hook_requirements() dependency in .install file

In the node_subpages install file, the requirements hook calls a function from the main module (node_subpages.module) itself. The main module is not loaded at install time, yielding a function not found error. This can be solved by loading the main module file manually:

function node_subpages_requirements($phase) {
  $t = get_t();
  $requirements = array();
  module_load_include('module', 'node_subpages');
  if (!node_subpages_check_ctools_version()) {
  ...