Test the fake derivations channel, asserting nested packages are properly represented.

This is a breaking change. Previously, packages named `packageset.foo`
would be exposed in the fake derivation channel as `packageset-foo`.

Presumably this was done to avoid needing to track attribute sets, and
to avoid the complexity. I think this now correctly handles the
complexity and properly mirrors the input expressions layout.

t/jobs/nested-attributes.nix

@@ -0,0 +1,36 @@
+with import ./config.nix;
+rec {
+  # Given a jobset containing a package set named X with an interior member Y,
+  # expose the interior member Y with the name X-Y. This is to exercise a bug
+  # in the NixExprs view's generated Nix expression which flattens the
+  # package set namespace from `X.Y` to `X-Y`. If the bug is present, the
+  # resulting expression incorrectly renders two `X-Y` packages.
+  packageset = {
+    recurseForDerivations = true;
+    deeper = {
+      recurseForDerivations = true;
+      deeper = {
+        recurseForDerivations = true;
+
+        nested = mkDerivation {
+          name = "much-too-deep";
+          builder = ./empty-dir-builder.sh;
+        };
+      };
+    };
+
+    nested = mkDerivation {
+      name = "actually-nested";
+      builder = ./empty-dir-builder.sh;
+    };
+
+    nested2 = mkDerivation {
+      name = "actually-nested2";
+      builder = ./empty-dir-builder.sh;
+    };
+  };
+  packageset-nested = mkDerivation {
+    name = "actually-top-level";
+    builder = ./empty-dir-builder.sh;
+  };
+}