Clean up parsing/writing of cordovaDependencies

As far as I understand, cordovaDependencies exist in these places:

- On an entire package (not an individual unibuild), as stored in the
  main JSON blob (isopack.json)
- On a target (combination of many packages), as stored in the star

This commit removes some stray code where they show up in other places:

- A field (which is never written) is read from the individual unibuild
  JSON file and passed to the Unibuild constructor, which does not
  expect it.
- A field (which is never read) is written to the buildinfo.json file.

Also, an assignment to Isopack.cordovaDependencies is pulled out of a
loop.

@Slava Can you confirm that these changes are correct?