[Debug] fixed class lookup when using PSR-0 with a target dir
minor #12071 [Debug] fixed class lookup when using PSR-0 with a target dir (fabpot)

This PR was merged into the 2.4 branch.

Discussion
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[Debug] fixed class lookup when using PSR-0 with a target dir

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
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f5b38ec [Debug] fixed class lookup when using PSR-0 with a target dir