Add search for packages in the Package Registry

Proposal

Those who do not know which packages are available in the GitLab Package Registry do not have a means to find out without manual documentation. That is, packages in the Registry need discoverability. This is especially an issue if there are many packages or they are used by different teams in an organization. I propose adding a Packages page with search. Ideally, the Packages page would contain filters based on package type, e.g. npm.

The package search should also be integrated with the main search, displaying results in a tab there.

The first step to making this happen would probably be to add an API endpoint for listing packages for the GitLab instance, instead of just by project or group.