pw_protobuf_compiler: Depend on Python package

When pw_proto_library Python packages are nested in another package,
have the .python subtarget wrap the package in which they're nested.
pw_proto_library targets originally disabled the .python subtarget when
the proto library was nested in another Python package. This made having
one pw_proto_library target depend on another difficult, and created
an error condition that didn't need to exist.

Change-Id: I53fe712d21f9a3d3100b0c3c395abc60fa2c666a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/47760
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>

pw_protobuf_compiler/docs.rst

diff --git a/pw_protobuf_compiler/docs.rst b/pw_protobuf_compiler/docs.rst
index 99e3d18..91c3310 100644
--- a/pw_protobuf_compiler/docs.rst
+++ b/pw_protobuf_compiler/docs.rst
@@ -97,6 +97,7 @@
 * ``strip_prefix``: Remove this prefix from the source protos. All source and
   input files must be nested under this path.
 * ``python_package``: Label of Python package to which to add the proto modules.
+  The .python subtarget will redirect to this package.
 
 **Example**
 

pw_protobuf_compiler/proto.gni

diff --git a/pw_protobuf_compiler/proto.gni b/pw_protobuf_compiler/proto.gni
index 07f4d52..241c666 100644
--- a/pw_protobuf_compiler/proto.gni
+++ b/pw_protobuf_compiler/proto.gni
@@ -308,18 +308,11 @@
   }
 
   if (defined(invoker.python_package) && invoker.python_package != "") {
-    # If anyone attempts to depend on this Python package, print an error.
-    pw_error(target_name) {
-      _pkg = get_label_info(invoker.python_package, "label_no_toolchain")
-      message_lines = [
-        "This proto Python package is embedded in the $_pkg Python package.",
-        "It cannot be used directly; instead, depend on $_pkg.",
-      ]
-    }
-    foreach(subtarget, pw_python_package_subtargets) {
-      group("$target_name.$subtarget") {
-        deps = [ ":${invoker.target_name}" ]
-      }
+    # This package is nested in another Python package. Depending on this
+    # its python subtarget is equivalent to depending on the Python package it
+    # is nested in.
+    pw_python_group(target_name) {
+      python_deps = [ invoker.python_package ]
     }
 
     # This proto library is merged into another package, but create a target to
@@ -375,6 +368,7 @@
 #   strip_prefix: Remove this prefix from the source protos. All source and
 #       input files must be nested under this path.
 #   python_package: Label of Python package to which to add the proto modules.
+#       The .python subtarget will redirect to this package.
 #
 template("pw_proto_library") {
   assert(defined(invoker.sources) && invoker.sources != [],