[1.0.0-alpha2] Rename Icon packs and UIIcon

Jean: "Packs are the icon files, as provided by upstream. Sets are the UI artefact we are defining and manipulating".

We prefer "Icon pack".

Let's also change the project page

I am nitpicking, but while we renaming "icon set" to "icon pack" in the filenames, is it possible to also remove the "UI" prefix?

• Drupal\ui_icons\Plugin\UiIconsetManagerInterface >> Drupal\ui_icons\Plugin\IconpackManagerInterface
• Drupal\ui_icons\Plugin\UiIconsetManager >> Drupal\ui_icons\Plugin\IconpackManager
• Drupal\ui_icons\PluginForm\UiIconsetExtractorForm >> Drupal\ui_icons\PluginForm\IconpackExtractorForm

Because we are already in the ui_icons namespace, no need to repeat it in filename, and it seems Drupal Core doesn't do such a thing.

(I will create another ticket (alpha3? later?) after this one to remove "UiIcon" in other filenames.)

What about advance on this task for a global renaming and usage of Icon instead of UiIcons.

Fast merging as there is way too much changes to isolate testing.