contributed modules can't use install phase of hook_requirements

Does the above change work for you?

No, still nothing is returned, I guess you meant something like "$module.install$" instead of "^modules/$module.install$".

But anyway, I think usage of a bare file_scan_directory() won't cut it because several directories like 'modules/', 'sites/all/modules/' and 'sites/example.com/modules/' need to be searched and some directories like 'sites/otherexample.com/modules/' should not be searched.
The function drupal_system_listing() is very handy here to search through the right directories:

function drupal_get_install_files($module_list = array()) {
  $installs = array();
  foreach ($module_list as $module) {
    $installs = array_merge($installs, drupal_system_listing("$module.install$", 'modules', 'name'));
  }
  return $installs;
}

This works nicely.
see attached patch

at #3:

I think the min_depth=0 in the original drupal_get_install_files() implementation is a leftover from the time (drupal 4.x) that modules did not reside in their own directory. But since Drupal 5 that's not the case anymore and it is considered good style to put each module in its own directory. That's why min_depth of drupal_system_listing() defaults to 1, I guess. drupal_system_listing() was introduced in Drupal 5, while file_scan_directory() goes back to D4.6.

Considering argument defaults, even the $key='name' argument can be dropped, because that's the default:

function drupal_get_install_files($module_list = array()) {
  $installs = array();
  foreach ($module_list as $module) {
    $installs = array_merge($installs, drupal_system_listing("$module.install$", 'modules'));
  }
  return $installs;
}

see attached patch

at #3:

I think the min_depth=0 in the original drupal_get_install_files() implementation is a leftover from the time (drupal 4.x) that modules did not reside in their own directory. But since Drupal 5 that's not the case anymore and it is considered good style to put each module in its own directory. That's why min_depth of drupal_system_listing() defaults to 1, I guess. drupal_system_listing() was introduced in Drupal 5, while file_scan_directory() goes back to D4.6.

Considering argument defaults, even the $key='name' argument can be dropped, because that's the default:

function drupal_get_install_files($module_list = array()) {
  $installs = array();
  foreach ($module_list as $module) {
    $installs = array_merge($installs, drupal_system_listing("$module.install$", 'modules'));
  }
  return $installs;
}

see attached patch

(the first submission of this post failed in some strange way, I hope this is not a duplicate post)

at #3:

I think the min_depth=0 in the original drupal_get_install_files() implementation is a leftover from the time (drupal 4.x) that modules did not reside in their own directory. But since Drupal 5 that's not the case anymore and it is considered good style to put each module in its own directory. That's why min_depth of drupal_system_listing() defaults to 1, I guess. drupal_system_listing() was introduced in Drupal 5, while file_scan_directory() goes back to D4.6.

Considering argument defaults, even the $key='name' argument can be dropped, because that's the default:

function drupal_get_install_files($module_list = array()) {
  $installs = array();
  foreach ($module_list as $module) {
    $installs = array_merge($installs, drupal_system_listing("$module.install$", 'modules'));
  }
  return $installs;
}

see attached patch

(the first submissions of this post failed in some strange way, I hope this is not a duplicate post)

sorry about the duplicate posts :)

Since I'm not very familiar with Drupal core development, what would you suggest?

moving drupal_system_listing() to install.inc?
including common.inc earlier?
emulating drupal_system_listing in drupal_get_install_files()?
something else?