Use a helper function to get the requirement key

Change-Id: I05a3a664570a7ce058e3efda3089a56886ee59ce
Move multipip parsing and validate core requirements

Move the multipip output parsing function to its own small
utility function and also add in validation that the key core
packages that we are building are valid replacements when those
packages are found in requirement files (or abort).

Change-Id: Ic9598bb9b326097b79dd6bc32324d862a5cf9fbd
Merge "Move multipip parsing and validate core requirements"