Namespaced extras

It would be useful (at least it would be for me) if it were possible for extras added to packages to be namespaced.

I'm not sure whether this would be some form of extra field, or a convention (such as namespace:extra_name) but it would be useful for:
- Avoiding clashes between plugins with extras names.
- Being able to retrieve all the extras just from a specific namespace - think a plugin that adds extras outside the normal workflow.
- Allowing namespaced extras to be missing during an update without overwriting them - a plugin that changes extras and doesn't want those extras on the web form for instance.
- Deleting extras en masse.

If a convention were used I think it would be possible to introduce without significant rework to the current system (it would be a matter of parsing the name), but this may be error prone.

Start by mapping how package extras are stored, updated, exposed in the web form, and handled by plugins; no source files or tests are named in the issue. Compare an extra-field design with a naming convention, then define behavior for namespace lookup, missing extras during updates, and bulk deletion before identifying the affected tests.

Written by the indexing model from the issue text.