fetchtorrent: add flatten argument for rqbit

Currently, the output from fetchtorrent will be different depending on
whether the default "transmission" backend or the "rqbit" backend is
used, because "rqbit" changed its behaviour in v6.0.0 to create
subdirectories.

Restore the old behaviour for the rqbit backend of flattening the
directory structure, but add a "flatten" argument to allow users to
explicitly request the behaviour without this change.

Because fetchtorrent produces fixed-output derivations, it's possible
that people won't notice the changes in behaviour here, so add a warning
that the behaviour might be unexpected (in either direction!) if the
flatten argument isn't specified for the rqbit backend, and -- to avoid
needing to support it indefinitely -- a warning that `flatten = false`
will be deprecated in future.

Update the fetchtorrent tests to check all the relevant combinations,
and to mark all tests as now working.

Update the release notes to advertise this breaking change.

Fixes #432001.

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