Process plugin for importing/creating files by URL

I need this on 8.1.x for my current project.

Well, it turned out I didn't need it for that project in April - but I do for this week's project, so I'll be seeing if I can complete this patch within the next day.

A big piece missing here is actually copying the file into the files directory. Drupal 8.2.x should be introducing a general-purpose file_copy process plugin for this in #2695297: Refactor EntityFile and use process plugins instead - I'm thinking that the best solution here would be to add that plugin to migrate_plus in 8.x-2.x (which will be compatible with core 8.1.x only), and remove in 8.x-3.x (which will be compatible with core 8.2.x and up). Maybe add the urlencode process plugin from that patch as well...

I think if we do #2765733: entity_lookup should support file/image destination fields, we won't need the file_generate process plugin, just the file_copy plugin.

Yes, a combination of #2765733: entity_lookup should support file/image destination fields and the file_copy plugin does the job without a specialized generation plugin. Here's what the relevant parts of my migration look like to leverage this:

source:
...
  constants:
    picture_directory: public://profile_images/
process:
...
  # From https://www.example.com/profile_images/9629.jpg, generate a
  # destination_basename of 9629.jpg...
  destination_basename:
    plugin: callback
    callable: basename
    source: image_url
  # ...and on to a destination_path of public://profile_images/9629.jpg.
  destination_path:
    plugin: concat
    source:
      - 'constants/picture_directory'
      - '@destination_basename'
  field_picture:
    -
      plugin: skip_on_empty
      method: process
      source: image_url
    -
      plugin: file_copy
      source:
        - image_url
        - '@destination_path'
    -
      plugin: entity_generate

Sigh... Thought I had committed this, so I though it was in beta2...