PmCheck: Big refactor of module structure

  * Move everything from `GHC.HsToCore.PmCheck.*` to
    `GHC.HsToCore.Pmc.*` in analogy to `GHC.Tc`, rename exported
    `covCheck*` functions to `pmc*`
  * Rename `Pmc.Oracle` to `Pmc.Solver`
  * Split off the LYG desugaring and checking steps into their own
    modules (`Pmc.Desugar` and `Pmc.Check` respectively)
  * Split off a `Pmc.Utils` module with stuff shared by
    `Pmc.{,Desugar,Check,Solver}`
  * Move `Pmc.Types` to `Pmc.Solver.Types`, add a new `Pmc.Types` module
    with all the LYG types, which form the interfaces between
    `Pmc.{Desugar,Check,Solver,}`.