Fix issue resolving local path dependencies
Fix issue resolving local path dependencies

Local path dependencies sometimes (usually) failed to resolve. There
turned out to be an issue when processing the cargo file where if a
dependency didn't have a `path` attribute, the get_local_packages
function returned None. Now, the function only returns None when there
is no parent directory for the cargo file.

Resolves #529.

alkama/racer,jaxx/racer,phildawes/racer,racer-rust/racer,sliz1/racer,hotpxl/racer,birkenfeld/racer,pmarcelll/racer