Loading tarballs with content permissions set to non-readable

Some package are tarballs with weird permissions. For example, this one has no permission bit set at all. This makes the loader fail while loading, then fail again while cleaning up its temporary directory, with PermissionError.

We should at least fix the clean up part, but ideally we should make the loading work.

Sentry issue: SWH-LOADER-CORE-A3

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Migrated from T2558 (view on Phabricator)