handle transitive deps of additional_link_libraries

When defining an `additional_link_library` that uses `deps`, e.g.

```starlark
cc_library(
    name = "start",
    srcs = ["start.c"],
    ...
    deps = [":registers"],
)
```

where the `start` library depends on a separate `registers` library,
libraries in `deps` are not compiled. Similar to
`additional_linker_inputs`, these files must be explicitly forwarded in
the DefaultInfo returned by the transition implementation function.

Author: oliverlee

MODULE.bazel.lock

@@ -46,12 +46,19 @@ arm_none_eabi_toolchain(
     ],
 )
 
+# this dummy is used to test that transitive deps are included
+cc_library(
+    name = "start_dep",
+    srcs = ["dummy.c"],
+)
+
 # always linked in with the toolchain below
 cc_library(
     name = "start",
     srcs = ["start.c"],
     additional_linker_inputs = ["link.ld"],
     linkopts = ["-T $(location :link.ld)"],
+    deps = [":start_dep"],
 )
 
 # Cortex-M4 toolchain that always links `:start`

examples/bzlmod/custom/toolchain/BUILD

@@ -0,0 +1 @@
+void dummy() {}

examples/bzlmod/custom/toolchain/dummy.c

@@ -14,13 +14,26 @@ _toolchain_transition = transition(
 )
 
 def _toolchain_transition_library_impl(ctx):
+    to_list = lambda x: [x] if x else []
+
     default_info = ctx.attr.src[DefaultInfo]
     cc_info =  ctx.attr.src[CcInfo]
 
     linker_input_files = []
+
+    # We need to recreate default_info since not all files are propagated with
+    # the transition. Specifically, the following are known to not propagate:
+    # * additional_linker_inputs (e.g. linker scripts)
+    # * deps (i.e. other libraries this one depends on)
     for linker_input in cc_info.linking_context.linker_inputs.to_list():
         linker_input_files.extend(linker_input.additional_inputs)
 
+        for lib in linker_input.libraries:
+            linker_input_files.extend(to_list(lib.static_library))
+            # I'm not sure if these below are needed
+            linker_input_files.extend(to_list(lib.pic_static_library))
+            linker_input_files.extend(to_list(lib.dynamic_library))
+
     return [
         DefaultInfo(
             files = depset(