Always use label of LinkableGraph for traversal

Summary:
D65716309 added an alias for a cpp_library as a dep instead of a cpp_library directly and that broke `haskell_ghci` rules with the following error:
```
Error running analysis for `fbcode//common/hs/thrift/exactprint:ghci (cfg:linux-x86_64-fbcode-platform010-clang17-asan-ubsan-dev#f0bd86d3824be5aa)`

Caused by:
    Traceback (most recent call last):
      File <builtin>, in <module>
      * fbcode/buck2/prelude/haskell/haskell_ghci.bzl:691, in haskell_ghci_impl
          omnibus_data = _build_haskell_omnibus_so(ctx)
      * fbcode/buck2/prelude/haskell/haskell_ghci.bzl:191, in _build_haskell_omnibus_so
          all_nodes_to_exclude = depth_first_traversal(
      * fbcode/buck2/prelude/utils/graph_utils.bzl:186, in depth_first_traversal
          return depth_first_traversal_by(graph_nodes, roots, lookup)
      * fbcode/buck2/prelude/utils/graph_utils.bzl:237, in depth_first_traversal_by
          fail("Expected node {} in graph nodes".format(node_formatter(node)))
    error: fail: Expected node fbcode//tools/build/sanitizers:fbcode-sanitizer-cpp (cfg:linux-x86_64-fbcode-platform010-clang17-asan-ubsan-dev#f0bd86d3824be5aa) in graph nodes
       --> fbcode/buck2/prelude/utils/graph_utils.bzl:237:13
        |
    237 |             fail("Expected node {} in graph nodes".format(node_formatter(node)))
        |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
```

The issue is that this graph traversal code confuses labels encoded in LinkableGraph objects (which would use the label of the aliased target) with labels of deps (which would use the label of the alias directly). Fix this by ensuring that this code consistently looks at labels of LinkableGraph objects.

Reviewed By: iguridi

Differential Revision: D65989756

fbshipit-source-id: 2fcc30fad197bfdf0f85975ab82f2da57567a2d4

haskell/haskell_ghci.bzl

@@ -190,7 +190,7 @@ def _build_haskell_omnibus_so(ctx: AnalysisContext) -> HaskellOmnibusData:
     # Need to exclude all transitive deps of excluded deps
     all_nodes_to_exclude = depth_first_traversal(
         dep_graph,
-        [dep.label for dep in preload_deps],
+        [dep[LinkableGraph].label for dep in preload_deps if LinkableGraph in dep],
     )
 
     # Body nodes should support haskell omnibus (e.g. cxx_library)