Fix components in build-type: Custom packages (#2257)

* Fix components in `build-type: Custom` packages

These components may depend on the `library` component of the package however `plan.json` does not include that dependency (since cabal will build all the components at once).  This fix adds the library component as a dependency of the other components.

* ifdLevel 1

* Add comment and only include library if there is one.

* ifdLevel 2

* ifdLevel 3

* Fix broken test

lib/load-cabal-plan.nix

@@ -47,7 +47,15 @@ let
               name = pkgs.lib.removePrefix "${prefix}:" n;
               value = (if cabal2nixComponents == null then {} else cabal2nixComponents.${collectionName}.${name}) // {
                 buildable = true;
-              } // lookupDependencies hsPkgs c.depends c.exe-depends;
+              } // lookupDependencies hsPkgs (
+                    c.depends
+                    # If plan.json uses a single unit for this package (build-type: Custom),
+                    # then it will leave the package itself out of `c.depends` for the
+                    # components of the package.
+                    # Haskell.nix builds the components separately so we need
+                    # to add the `library` component as a dependency.
+                    ++ pkgs.lib.optional (p ? components && p.components ? lib) p.id
+                  ) c.exe-depends;
             in { inherit name value; }
           )) components));
     in

test/setup-deps/pkg/src/Pkg.hs

@@ -0,0 +1,4 @@
+module Pkg where
+
+foo :: Int
+foo = 1