Rename "usings" property and make it do what the documentation says.

- This property only considered target artifacts from dependencies,
  which is not what the documentation says.
- The name "usings" has long been known to suck badly. Call it
  "inputsFromDependencies" instead. This name now fits (see above).

Change-Id: I3aab11ba1446da3ba23f8760eed6fca973b971ca
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>