[Debug] fix #10771 DebugClassLoader can't load PSR4 libs
bug #10776 [Debug] fix #10771 DebugClassLoader can't load PSR4 libs (nicolas-grekas)

This PR was merged into the 2.5-dev branch.

Discussion
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[Debug] fix #10771 DebugClassLoader can't load PSR4 libs

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10771
| License       | MIT
| Doc PR        | none

Commits
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e3566c2 [Debug] fix #10771 DebugClassLoader can't load PSR4 libs