Move SolverInstallPlan to its own module.
This is a preparatory commit for giving SolverInstallPlan its own type. We first start by moving the type synonyms for SolverInstallPlan into their own module, and update module references to point to them. TODO: Maybe this module should go in the Solver hierarchy rather than the client hierarchy? Signed-off-by: Edward Z. Yang <email@example.com>