Virtual Maven registries: API endpoints code organization

🔥

Extraction from Maven virtual registry: technical debts (#480360 - closed):

lib/api/virtual_registries/packages/maven.rb code organization. See !162614 (comment 2066648290).

Also see this thread.

🚒 Solution

We're currently exploring:

• Registry C + R(list) under Group. (we don't have Registry R(list) today).
• Registry R(single) + UD as standalone.
• Upstream C + R(list) under Registry.
• Upstream R(single) + UD as standalone.
• Cached Response R(list) under Upstream.
• Cached Response D as standalone.

In other words: "if we access the resources by its primary key, then the endpoint is located at the root (standalone) + create (+list) endpoint where it belongs"