What do I have to do to get hook_form_alter picked up? Trying to move from Drupal 7 to Drupal 8.  First step is to get a custom module with hook_form_alter() and the Devel module working so that I can see how bad I am messing things up.

To do so, I made a custom module with the following files.

mymodule.info.yml

name: My Module
type: module
description: "Custom module"
package: Custom
core: 8.x


mymodule.module

&lt;?php

function mymodule_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  dpm($form);
  print_r($form);
}


Other steps


I enabled both the Devel module and this custom module.
I gave anonymous users permission to view devel information.
I made sure the Status block was enabled on the Block Admin page.
I cleared all caches via the Pantheon admin interface.
I tried using print_r($form), dpm($form), and var_dump($form).


However, when I refresh /user/login, I don't see any output from dpm(), so the hook isn't getting picked up for some reason.  What else do I need to do?
It turns out this was something related to MultiDev, a Pantheon-specific feature.  By cloning back and forth between the main dev branch and my multidev branch, I was able to get the custom hook_form_alter() to work.