Improve the package upload workflow

[quba42]

Is your feature request related to a problem? Please describe.

The current upload workflow is not very user friendly.

Users have a .deb file, and want to add that Package to some APT repo in Pulp. Users do not care about how Pulp turns that File into deduplicated artifacts and content units.

Describe the solution you'd like

Using this API endpoint We want to achieve all of the following:

• If there is already an artifact for the file I upload, then use it to create the content unit.
• If there is already a content unit, then use that content unit and add it to the repo.
• Do not allow users to provide their own relative_path, only to reject it as "not how pulp_deb likes it".
• Provide the ability to add the package to an existing ReleaseComponent in the repo. (Perhaps split this out in a follow on task)

[quba42]

See also:

• [EPIC] Unified API endpoint to determine if a file can be uploaded without error pulpcore#2911
• https://github.com/pulp/pulp_file/issues/774

[quba42]

There is now a pulpcore feature that plugins can opt in to use: pulp/pulpcore#3083