Merge requirements and dependencies fields in plugins_index

[MarkHedleyJones]

There is some level of ambiguity around what the difference between "requirements" and "dependencies" are in the plugin-index page.
This is partly because of the transition to symantic versioning which has left things in limbo.

It would be good to merge these two fields and in the process, add the ability to specify versions for the packages that are currently defined under "dependencies"