In cmTarget, cmGeneratorTarget, and cmGeneratorExpressionEvaluator, fix
target name lookups to occur in the cmMakefile context of the target
that referenced the name, not the current 'head' target.  The context
matters for imported targets because they are directory-scoped instead
of globally unique.  We already do this in cmComputeLinkDepends and
cmComputeTargetDepends.

Extend the InterfaceLibrary test with an example covering this behavior.