[CODE TASK] Update all modules to `drupal/` namespace in Composer

[rosiel]

We have moved our modules to drupal.org, though they are still available through packagist. We ought to harmonize our modules to use one namespace (drupal/), and take care of this so it's a smooth transition.

Steps/suggestions:

• Change composer.json's top-level name: key to use the drupal namespace.
• Add a replace key that includes the old namespace, at all versions. This way things other packages that require islandora/islandora can be satisfied by drupal/islandora.
• Update packagist to mark our packages as abandoned(?!!), and suggest the new namespace.

I want there to be little fuss and so I'm looking for feedback on what to do.

[rosiel]

The above PRs do not close this issue. As decided in Islandora/islandora#1020, we're going to keep this open for 3 months to provide a buffer time after the deprecation notice. Then, we'll actually change the main namespaces in the composer.json files. That'll make everything easier to use.