[INTERNAL|API] ChecksumCache requires IFiles instead of (String) paths

Should make it a little bit less error prone as long as the interface
are correctly implemented.

Change-Id: I46dca86cd276f3e48b2917e189ed35e73b01b080
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1934
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>