[init] Changes the check used to determine if a Podfile already exists.

Previously init would check to ensure that the current config did not have a Podfile.
This presented issues in cases where the init command was run in a directory who's parent had a Podfile because config will search its parent if one is not found in the present directory.
Fixed the issue by exposing `podfile_path_in_dir` and using it in the init command to test if the pwd had a Podfile. This is the behaviour that was being tested for in spec/functional/command/init_spec.rb:26.

Fixes #2181