pw_env_setup: Use GN to install Python packages

For now leave the setup.py search in place to make migration easier for
downstream projects, but largely turn it off for Pigweed itself.

Bug: 287
Change-Id: I79e3a1ba64f98eec2b6617903fad634e8ef0b95a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/23160
Commit-Queue: Rob Mohr <mohrr@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
diff --git a/pw_build/python.gni b/pw_build/python.gni
index 6b523ca..9ca3a43 100644
--- a/pw_build/python.gni
+++ b/pw_build/python.gni
@@ -26,7 +26,7 @@
 #     - $name.lint.mypy - Runs mypy.
 #     - $name.lint.pylint - Runs pylint.
 #   - $name.tests - Runs all tests for this package.
-#   - $name.install - Installs the package in a venv. (Not implemented.)
+#   - $name.install - Installs the package in a venv.
 #   - $name.wheel - Builds a Python wheel for the package. (Not implemented.)
 #
 # TODO(pwbug/239): Implement installation and wheel building.