Change file disposition code to access rpmfi's by index

Change file disposition code to access rpmfi's by index

- The final pre-requisite to handling file conflicts within a package:
  with this we're no longer tied to the single index per rpmfi. This
  is not supposed to change anything yet unless I screwed something up.
  Also goes to show that a semi-iterator interface for something
  that really needs random access only gets in the way rather than
  helping...