Crawler default namespace fix
bug #9771 Crawler default namespace fix (crudecki)

This PR was squashed before being merged into the 2.4 branch (closes #9771).

Discussion
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Crawler default namespace fix

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9732, #6650
| License       | MIT
| Doc PR        | symfony/symfony-docs/pull/2979

Fix backwards compatibility of xml namespaces for having only one default namespace.

Commits
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cfff054 Crawler default namespace fix
fixed CS