urlget: fix download command

Destination directory was missing.

So this was not working. I implemented this while I was offline, and I
forgot to test later that it was really working.
Store created input_files to output directory

It used to be created in each project's directory, but it is better to
have them in output directory, to remove them from 'git status'.

For 'project' input_files, the files are stored in the output_dir of
that project.

If you want to store some file in a different directory, it is possible
to override the 'output_dir' option in the input_file definition.