I have a suspicion that MDL-53978 caused a regression. Those calls like

get_plugins_with_function('before_http_headers', 'lib.php');

are also made during plugin installation (when rendering the plugin checks screen) and they cause lib.php files be loaded from all plugins - even if the plugin may have an unsatisfied dependency.

Reproduce:
1. Have a plugin B that correctly declares dependency on a plugin A. In the B's lib.php, require_once() some A's file.
2. Unzip the code of the plugin B into Moodle and run installation by visiting the admin main screen

Expected:
The missing dependency A should be reported, giving the admin the opportunity to provide it

Actual:
PHP fatal error thrown because B's lib.php has been loaded prematurely to check for the presence of the new callbacks.