GraphQL: update search behavior to prioritize last item in repo path and customize behavior for colon

[andaaron]

From one of the UX discussions:

1. UI search: always return "image" names

alpine => [a/b/alpine, alpine, xyz/alpine]

Don't return image versions.

2. UI search: "image" followed by a colon, returns "image versions" list, sorted chrono descending

alpine: => [alpine:1, alpine:2, ...]

3. Do not return layers for now - let's have a separate call about searching for layers later.