Support strict build dependencies

Since the beginning when we originally introduced non strict mode, we knew that the "strict rebuild" flag we used for elements like compose (which literally consume the input) would not be enough (not able to fine tune non-strict build plans for cases where some libraries are statically linked, for instance).

This implements a strict flag on the dependency dictionary for the cases one might want to declare a dependency as strict, and addresses #254 (closed) in doing so.