Add a new Cmd type working on RealisedPaths

Where a `RealisedPath` is a store path with its history, meaning either
an opaque path for stuff that has been directly added to the store, or a
`Realisation` for stuff that has been built by a derivation

This is a low-level refactoring that doesn't bring anything by itself
(except a few dozen extra lines of code :/ ), but raising the
abstraction level a bit is important on a number of levels:

- Commands like `nix build` have to query for the realisations after the
  build is finished which is fragile (see
  27905f12e4a7207450abe37c9ed78e31603b67e1 for example). Having them
  oprate directly at the realisation level would avoid that
- Others like `nix copy` currently operate directly on (built) store
  paths, but need a bit more information as they will need to register
  the realisations on the remote side

1 files changed, 2 insertions, 0 deletions

diff --git a/src/nix/copy.cc b/src/nix/copy.cc
index f15031a45..c56a1def1 100644
--- a/src/nix/copy.cc
+++ b/src/nix/copy.cc
@@ -16,6 +16,8 @@ struct CmdCopy : StorePathsCommand
 
     SubstituteFlag substitute = NoSubstitute;
 
+    using StorePathsCommand::run;
+
     CmdCopy()
         : StorePathsCommand(true)
     {