Allow packages to be compared to sibling packages

[ChristopherFry]

Feature Suggestion

Allow packages to be compared with sibling packages (packages that share the same upstream package). Today, shown in the screenshot below, packages can only be compared to earlier revisions and their upstream revision.

Possible Implementation

A possible implementation is to list sibling revisions in the Compare Revision dropdown as 'Sibling ([package-name] [package-revision])'. These should show after the upstream revision in the dropdown. If there are multiple siblings with the same package name (possible if they are cloned to separate repositories), the package name can be prefixed with the repository name. Only the latest published package revision for each sibling needs to be shown in the dropdown.

Context

Comparing siblings allows differences to be quickly and easily viewed between variants. Today this isn't possible with the UI.

[ChristopherFry]

Adding good first issue as this should be a good issue for a new comer to take on.

The easiest way to create sibling packages is to follow the Namespace Provisioning UI Guide and create the 'simplens' team blueprint, and from the team blueprint, clone two deployments packages, 'frontend' and 'backend', and publish the first revision of each of these packages. Here, the 'frontend' and 'backend' will be siblings of each other, and the expectation is when viewing one of these deployments, you should be to compare it with the other deployment package using the 'Compare Revision' dropdown on the Package Revision screen.