Downloader: Protect against partially downloaded files.

[modmuss50]

Seen one or two users on Discord reporting loom only downloading parts of the file, I think this is quite rare. This change should protect against failed or cancled downloads by ensuring that the output file only exists in its final state.

1. Download the file initially to a <filename>.part file
2. Once the file has been fully written, create a hard link to the destination file.
3. And then remove the part file, this ensures that the output file only exists in fully populated state.