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/bootstrap.sh b/bootstrap.sh
index 246d007..a7fdd5f 100644
--- a/bootstrap.sh
+++ b/bootstrap.sh
@@ -69,6 +69,10 @@
 PW_ROOT="$(dirname "$_BOOTSTRAP_PATH")"
 export PW_ROOT
 
+# Please also set PW_PROJECT_ROOT to YOUR_PROJECT_ROOT.
+PW_PROJECT_ROOT="$PW_ROOT"
+export PW_PROJECT_ROOT
+
 . "$PW_ROOT/pw_env_setup/util.sh"
 
 pw_deactivate