Discard duplicate operations to copy files to snapshot

Header include directories are copied into the snapshot separately for
each cc library that exports them. However, the same include
directories can be exported by multiple libraries which caused ninja
error because two separate rules (albeit identical) were defined to
create the same files.

This avoids the duplicate ninja rules by detecting and discarding
duplicate copies, i.e. where the source and destination are the same.
It will also report an error if two or more different source files are
copied to the same destination.

Bug: 142918168
Test: m nothing
      added test and verified it produced two identical copy rules
      fixed code and verified duplicate copy rules had been eliminated

Change-Id: I39e37405035bee5093f96e03248e9e29ed30962c