Replace `PubGrubDependencies` by `PubGrubDependency` (#4481)
In the last PR (#4430), we flatten the requirements. In the next PR
(#4435), we want to pass `Url` around next to `PubGrubPackage` and
`Range<Version>` to keep track of which `Requirement`s added a url
across forking. This PR is a refactoring split out from #4435 that rolls
the dependency conversion into a single iterator and introduces a new
`PubGrubDependency` struct as abstraction over `(PubGrubPackage,
Range<Version>)` (or `(PubGrubPackage, Range<Version>,
VerbatimParsedUrl)` in the next PR), and it removes the now unnecessary
`PubGrubDependencies` abstraction.
