Avoid the possibility of treating a dir named composer as a file.

In case a path in the `PATH` environment variable contains a directory
named `composer`, it might be use as the alleged composer "executable"
as `file_exists` returns `true` for directories.

src/Console/Command/Task/ProjectTask.php

@@ -121,7 +121,7 @@ protected function _searchPath($path) {
 		$composer = ['composer.phar', 'composer'];
 		foreach ($path as $dir) {
 			foreach ($composer as $cmd) {
-				if (file_exists($dir . DS . $cmd)) {
+				if (is_file($dir . DS . $cmd)) {
 					$this->_io->verbose('Found composer executable in ' . $dir);
 					return $dir . DS . $cmd;
 				}