Fix info file naming upon import
**Current behaviour:** Upon import, schnapps prepends a `@` to the info file's name, e.g., `@19.info` (`@19` is the subvolume's name). This makes it 'invisible' to schnapps, e.g., `schnapps list`.
**Expected behaviour:** A file name of `19.info`.
This issue was reported in the [forum](https://forum.turris.cz/t/schnapps-import-does-not-work/14474). Patch !15 should rectify it.
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