Resolve "Add plugins with new dependency model to hub API endpoints and yaml files"

To reduce change of slowing down New paradigm for bundling plugin dependencies a... (meltano#3433 - closed), I've drafted this MR for us to review.

Following from the discussion there in the above issue, requires and depends_on both to me seemed very explicit in terms of a strong dependency relationship. My problem with depends_on was that it did not strongly suggest that the plugin would be auto-installed - only that it would not function properly if the other was missing. I think requires conveys that the listed plugin will get auto-installed unless it is already present, which I feel matches our primary use case.

Partially resolves #251 (closed)