Update verdi import

[CasperWA]

As mentioned here:

Suggestions to update verdi import code (by @ltalirz):

• archives should simply be processed one by one (there is no need to create and validate the full list of archives before you even start. e.g. rm file1 file2 file3 also will start removing files and only stop when it encounters a file that isn't there)
• when you look at an archive you figure out whether it's a url or not. if url, validate URL and call function to download. you replace url by path to downloaded file and the following code runs inside a sandbox folder
• from now on all code is shared...

[chrisjsewell]

I think this was all addressed in #4532, but feel free to re-open if you think there is room for improvement 😄