ruby treewide: don't merge source into top-level

Keep the `source` attrset distinct to prevent its entries from merging
with the top level attrs.
Since each type of source has a different set of entries for `source`,
this is the easiest way to keep them together.
This will pave the way for a new `url` type of source.

This is a mass-rebuild of many ruby packages,
notably those using `git` type sources.