Code assumes that `installer-paths` elements are _always_ arrays

[diegoe]

The current code assumes that elements are always arrays:

drupal-finder/src/DrupalFinder.php

Lines 205 to 211 in 9f16035

	if (is_array($json)) {
	if (isset($json['extra']['installer-paths']) && is_array($json['extra']['installer-paths'])) {
	foreach ($json['extra']['installer-paths'] as $install_path => $items) {
	if (in_array('type:drupal-core', $items) ||
	in_array('drupal/core', $items) ||
	in_array('drupal/drupal', $items)) {

However, upstream "allows" simple strings by casting to array:

https://github.com/composer/installers/blob/2a9170263fcd9cc4fd0b50917293c21d6c1a5bfe/src/Composer/Installers/BaseInstaller.php#L116-L119

Predictably, this causes a crash:

PHP Fatal error:  Uncaught TypeError: in_array(): Argument #2 ($haystack) must be of type array, string given in  /webflo/drupal-finder/src/DrupalFinder.php:113
Stack trace:
#0 webflo/drupal-finder/src/DrupalFinder.php(113): in_array('type:drupal-cor...', 'a-module-name-im-using...')

Whether to follow upstream or not, it would be best to avoid the crash since strings instead of arrays is "not wrong" (?).