ENYO-3012: use `packagejs:` instead of `source:` in `deploy.json`

- the `source:` property of `deploy.json` did not make sense as it was
  always refering  to the  folder where `deploy.json`  is.  it  is now
  replaced by a `packagejs:`  property (default: `"package.js"`) which
  indicates  where   is  the   relative  location  of   the  top-level
  `package.js` from the project root folder.

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>