Extend DepGraph so it can track "work-products"

Extend DepGraph so it can track "work-products"

A work product right now is just a `.o` file. In the future it probably
includes other kinds of files, such as `.bc` files saving the
unoptimized LLVM IR.

However, because WorkProductIds must be independent of DefIds, so that
they don't need translation, this system may not be suitable *as is* for
storing fine-grained information (such as the MIR for individual defs),
as it was originally intended. We will want to refactor some for that.