There are two cases in which we replace source packages with ones that
are already installed: during planning when we improve the plan by
replacing configured source packages with pre-existing packages from the
store, but also there's just the case of skipping over a local package
that is already built and up to date. In the latter case we're replacing
a package in the Configured state not with one in the PreExisting state,
but with one in the Installed state, as if it had been built and
installed (which in a sense of course it has).

This new util is for the latter case.