When processing an external, the content of the target directory is
first removed before copying external dirs and files.
But that target directory might contain versioned paths so we must
ensure to restore them after removing the directory content.
Related to T611
Fixes some events in [SWH-LOADER-SVN-53](https://sentry.softwareheritage.org/share/issue/d6330b7c048249d99342e4c9d56dce19/)
Depends on D7087