[DIC] Move PrependExtensionInterface to the Extension namespace
merged branch vicb/di/prepext (PR #6853)

This PR was merged into the master branch.

Commits
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7944860 [DIC] Move PrependExtensionInterface to the Extension namespace

Discussion
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[DIC] Move PrependExtensionInterface to the Extension namespace

@fabpot Please merge before 2.2 (no BC break) /cc @lsmith77