Add support for disabling clean through target-prerequisite relationship

Our current semantics is to clean any prerequisites that are in the same
project (root scope) as the target and it may seem more natural to rather only
clean prerequisites that are in the same base scope. While it's often true for
simple projects, in more complex cases it's not unusual to have common
intermediate build results (object files, utility libraries, etc) reside in
the parent and/or sibling directories. With such arrangements, cleaning only
in base (even from the project root) may leave such intermediate build results
laying around (since there is no reason to list them as prerequisites of any
directory aliases). So we clean in the root scope by default but now any
target-prerequisite relationship can be marked not to trigger a clean with the
clean=false prerequisite-specific value.

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