Template files are now located in a templates subdirectory

Modules and themes both now default to having their templates in a templates sub-directory, for example:

mymodule/templates/foo.tpl.php

This can be overridden by setting the path key in hook_theme the same way as in Drupal 7:

function foo_theme() {
  return array(
    'foo' => array(
      'template' => 'foo.tpl.php',
      'path' => 'somewhere',
    )
  );
}

will cause the system to look for foo.tpl.php in the somewhere sub-directory.