Update FileLoader to fix issue #10339

FileLoader now uses resolved FileLoader's (the one that explicitly
supports that resource) FileLocatorInterface instance before falling
back to its own when trying to load resources in import() method.
bug #10348 Update FileLoader to fix issue #10339 (msumme)

This PR was merged into the 2.3 branch.

Discussion
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Update FileLoader to fix issue #10339

This fixes an issue in Symfony\Component\Config\Loader\FileLoader

| Q             | A
| ------------- | ---
| Bug fix?      |  yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |  #10339
| License       | MIT
| Doc PR        |  none

Commits
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3988728 Update FileLoader to fix issue #10339