ModuleHandler::add() removes implementations from other modules

Problem/Motivation

The method ModuleHandler::addModule() allows to add a module, and then ModuleHandler::add() attempts to add hook implementations for that module.
The methods were deprecated in #3481778: Deprecate functions using ModuleHandler::add(), and will be removed in Drupal 12.
We can assume that currently these method are rarely used, but their usage is still possible.

We already understand some limitations:
- Only procedural implementations are added.
- The resulting combined list of implementations is not properly ordered.

However, there is a bigger problem:
Calling ->add() removes existing implementations, and also prevents regular implementations from being added later.
Also, when ->resetImplementations() is called, the new implementations are lost.

This is much worse than simply not adding any new implementations.

Steps to reproduce

We can produce a KernelTest with code like this:

$module_handler = \Drupal::service(ModuleHandlerInterface::class);
// Assume that modules 'module_a' and 'module_b' are currently installed.
// Assume that implementations of test_hook return the respective module names.
$this->assertSame(['module_a', 'module_b'], $module_handler->invokeAll('test_hook'));
$module_handler->addModule('module_c');
$this->assertSame(['module_c'], $module_handler->invokeAll('test_hook'));  // ouch!!!
$module_handler->resetImplementations();
$this->assertSame(['module_a', 'module_b'], $module_handler->invokeAll('test_hook'));  // Implementation from module_c is forgotten.

One problem is the `->resetImplementations()` called within `->add()`.
We could remove this, but then it would still be problematic, if we call ->add() before ->invokeAll():

$module_handler = \Drupal::service(ModuleHandlerInterface::class);
// Assume that modules 'module_a' and 'module_b' are currently installed.
// Assume that implementations of test_hook return the respective module names.
$this->assertSame(['module_a', 'module_b'], $module_handler->invokeAll('test_hook'));
$module_handler->addModule('module_c');
// If we remove ->resetImplementations() from within ->add(), the old implementations are not lost.
$this->assertSame(['module_a', 'module_b', 'module_c'], $module_handler->invokeAll('test_hook'));  // nice, I guess..
$module_handler->resetImplementations();
$this->assertSame(['module_a', 'module_b'], $module_handler->invokeAll('test_hook'));  // Implementation from module_c is forgotten.

and

$module_handler = \Drupal::service(ModuleHandlerInterface::class);
// Assume that modules 'module_a' and 'module_b' are currently installed.
// Assume that implementations of test_hook return the respective module names.
// Assume that ->invokeAll('test_hook') was not called yet, so the list is not initialized.
$module_handler->addModule('module_c');
$this->assertSame(['module_c'], $module_handler->invokeAll('test_hook'));  // ouch!!!
$module_handler->resetImplementations();
// Now the list can be properly initialized.
$this->assertSame(['module_a', 'module_b'], $module_handler->invokeAll('test_hook'));

Proposed resolution

Remove module add chain of methods and just empty them.

Remaining tasks

Review

User interface changes

N/A

Introduced terminology

N/A

API changes

Technically - I can add a CR for this.

Data model changes

N/A

Release notes snippet

N/A