Make linking APIs aware of 'head' target

The 'head' is the dependent target to be linked with the current target.
It will be used to evaluate generator expressions with proper handling
of mapped configurations and is used as the source target of properties.

This requires that memoization is done with a key of a pair of target
and config, instead of just config, because now the result also depends
on the target.  Removing the memoization entirely is not an option
because it slows cmake down considerably.

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