When running `hugo convert` with `--output`, copy the content tree first so non-content bundle resources are kept in the destination, then overwrite converted content files.
Also avoid recursive self-copy when the output path points inside the content tree by skipping output directories during copy.
Fixes#4621
Primary motivation is documentation, but it will also hopefully simplify the code.
Also,
* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.
Closes#10896Closes#10620
Joe Mooring