Add lightweight 'autocomplete' API endpoint

[BenJam]

Use case: I am amending a package manifest in my favourite text-editor/IDE. I'd like to autocomplete the name and perhaps see a little information about the package if multiple options exist.

To enable someone to add this we might want a very small, fast API endpoint that returns an array of matched packages that can include a set of additional fields (a la the cdnjs API).

[andrew]

👍 Related to #124

[andrew]

Moving this to the Backlog as we'd still like to implement it but can't see that happening in the near future.