This restricts the solver to a closed set of packages.

Previously, even with freeze files, there was no good way to instruct
cabal-install to avoid pulling in extra, unconstrained
packages. `--reject-unconstrained-dependencies=all` forces the solver
to stay within the set of packages that are either local packages,
explicit goals, or which are explicitly constrained in configuration
files or via the `--constraint` flag

Closes #2568.