pw_build: Restructure as Python package

Change-Id: I2c0d2412f4ae6cd350591ad563931f9493bde6bd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/14720
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_build/exec.gni b/pw_build/exec.gni
index 3e51d57..9641f26 100644
--- a/pw_build/exec.gni
+++ b/pw_build/exec.gni
@@ -109,7 +109,7 @@
   }
 
   pw_python_script(target_name) {
-    script = "$dir_pw_build/py/exec.py"
+    script = "$dir_pw_build/py/pw_build/exec.py"
     args = _script_args
 
     forward_variables_from(invoker,