pw_protobuf_compiler: Wrap nanopb refs in Label

The reference to nanopb should be evaluated within the context of the
Pigweed repository, not the repository that uses nanopb_proto_library in
its BUILD.bazel file. Otherwise, users will be required to list nanopb
as a direct dependency in their MODULE.bazel (instead of using the
transitive dependency they get anyway through Pigweed).

Bug: 325533050
Change-Id: Ie10bcb4849c44a7537e7fed89fe020ed3dc86740
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/256660
Pigweed-Auto-Submit: Ted Pudlik <[email protected]>
Docs-Not-Needed: Ted Pudlik <[email protected]>
Lint: Lint 🤖 <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Reviewed-by: Dave Roth <[email protected]>

pw_protobuf_compiler/nanopb_proto_library.bzl

@@ -44,7 +44,7 @@ def nanopb_proto_library(*, name, deps, tags = [], options = None, **kwargs):
         name = name,
         protos = deps,
         deps = [
-            "@com_github_nanopb_nanopb//:nanopb",
+            Label("@com_github_nanopb_nanopb//:nanopb"),
             Label("//pw_assert"),
             Label("//pw_containers:vector"),
             Label("//pw_preprocessor"),
@@ -59,7 +59,7 @@ def nanopb_proto_library(*, name, deps, tags = [], options = None, **kwargs):
 
 _nanopb_proto_compiler_aspect = proto_compiler_aspect(
     ["pb.h", "pb.c"],
-    "@com_github_nanopb_nanopb//:protoc-gen-nanopb",
+    Label("@com_github_nanopb_nanopb//:protoc-gen-nanopb"),
     [],
     {
         "_pb_h": attr.label(