[DependencyInjection] Change visibility of YamlFileLoader methods to allow for custom overrides

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Target branch | **2.4**
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

#### Problem
1. Drupal uses `YamlFileLoader`, but Drupal does not use Symfony's Config component.
1. Various methods in `YamlFileLoader` are declared as `private` for no apparent reason.
1. This forced Drupal to [copy the entire class](https://github.com/drupal/drupal/blob/8.x/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php) instead of just extending it & overriding the necessary methods.
1. The code was copied from 2.3, Drupal upgraded to 2.4, people weren't aware of the literally copied code, and thus the copy became stale/outdated.

#### Proposed solution
1. Change the visibility of `YamlFileLoader` methods from `private` to `protected`.

To some extent, this is a continuation of #5981, which changed the visibility of (just) the `loadFile()` method for phpBB (which apparently seems to share some architectural aspects with Drupal).