pw_build: Fix pw_python_venv generated requirements

pw_python_venv was always adding the //pw_build/py gn target to the
list of dependencies to base the generated requirements on. This is
usually not an issue if pw_build is included. However if it isn't
needed extra 3p deps were always getting added from pw_build
regardless.

For example if a venv only needed //pw_docgen/py and //pw_status/py
the generated_requirements would include all these extra packages due
to pw_build being added in along with all their 3p deps:

  //pw_arduino_build/py:py
  //pw_build/py:py
  //pw_cli/py:py
  //pw_config_loader/py:py
  //pw_docgen/py:py
  //pw_env_setup/py:py
  //pw_package/py:py
  //pw_presubmit/py:py
  //pw_status/py:py
  //pw_stm32cube_build/py:py

This change fixes it so only these two are added:

  //pw_docgen/py:py
  //pw_status/py:py

Change-Id: I95c850c1532d70979acda23d84c0c47f8cbcad96
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/240792
Lint: Lint 🤖 <[email protected]>
Pigweed-Auto-Submit: Anthony DiGirolamo <[email protected]>
Docs-Not-Needed: Anthony DiGirolamo <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Reviewed-by: Wyatt Hepler <[email protected]>

pw_build/python_venv.gni

@@ -163,11 +163,10 @@ template("pw_python_venv") {
                             "($pw_build_PYTHON_TOOLCHAIN)" ]
       }
 
-      # pw_build/py is always needed for venv creation and Python lint checks.
-      python_metadata_deps =
-          [ get_label_info("$dir_pw_build/py", "label_no_toolchain") +
-            "($pw_build_PYTHON_TOOLCHAIN)" ]
-      python_metadata_deps += _pkg_gn_labels
+      # Add target packages to the python_metadata_deps. This will let
+      # GN expand the transitive pw_python_package deps which are read
+      # by generate_python_requirements.py
+      python_metadata_deps = _pkg_gn_labels
 
       args = [
         "--gn-root-build-dir",