feature #31310 [DependencyInjection] Added option ignore_errors: not_found for imported config files (pulzarraider)

This PR was merged into the 4.4 branch.

Discussion
----------

[DependencyInjection] Added option `ignore_errors: not_found` for imported config files

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#11647

If someone want to add optional config file. The only available choice was to add `ignore_errors: true` option

e.g.
```
imports:
    - { resource: parameters.yml, ignore_errors: true }
```

But this will hide all errors in imported file. We ran in many situations that broke our Symfony applications because we had a typo in this imported files.

This PR introduce new possible value `not_found` for `ignore_errors` option. It can be used for optional config files like the `ignore_errors: true`, but it will ignore only the file non-existence, not the possible syntax errors inside.

Usage:
```
imports:
    - { resource: parameters.yml, ignore_errors: not_found}
```

Commits
-------

e0ee01c10d [DependencyInjection] Added option `ignore_errors: not_found` while importing config files

Author: nicolas-grekas

Loader/FileLoader.php

@@ -73,7 +73,7 @@ public function getLocator()
      */
     public function import($resource, $type = null, $ignoreErrors = false, $sourceResource = null/*, $exclude = null*/)
     {
-        if (\func_num_args() < 5 && __CLASS__ !== \get_class($this) && __CLASS__ !== (new \ReflectionMethod($this, __FUNCTION__))->getDeclaringClass()->getName() && !$this instanceof \PHPUnit\Framework\MockObject\MockObject && !$this instanceof \Prophecy\Prophecy\ProphecySubjectInterface) {
+        if (\func_num_args() < 5 && __CLASS__ !== \get_class($this) && 0 !== strpos(\get_class($this), 'Symfony\Component\\') && __CLASS__ !== (new \ReflectionMethod($this, __FUNCTION__))->getDeclaringClass()->getName() && !$this instanceof \PHPUnit\Framework\MockObject\MockObject && !$this instanceof \Prophecy\Prophecy\ProphecySubjectInterface) {
             @trigger_error(sprintf('The "%s()" method will have a new "$exclude = null" argument in version 5.0, not defining it is deprecated since Symfony 4.4.', __METHOD__), E_USER_DEPRECATED);
         }
         $exclude = \func_num_args() >= 5 ? func_get_arg(4) : null;