Reorganize API endpoints

We have a lot of API endpoints exposed that are not used and/or unnecessary. These endpoints should be removed:
```
approximate_directory_content_index
approximate_directory_structure_index
exact_file_index
exact_package_archive_index
cditems
on_demand_queue
```

The matching related endpoints, `approximate_directory_content_index`, `approximate_directory_structure_index`, `exact_file_index`, and `exact_package_archive_index`, are being removed because the current D2D pipeline only uses sha1 lookups on the `packages` and `resources` endpoints. We also plan to revamp matching such that it is all done on the purldb server.

We should reorganize the API endpoints into three categories:

1. lookup (query, get)
2. match (post)
3. "mining" (live retrieval of things)

We also need to rethink the actions on the `packages` endpoint. The `get_or_fetch_package` endpoint should be merged with the `get_package` endpoint. `get_package` should also be updated to create a package from a download url, in addition to purls.