Use modern_python_path_defaults

The python path for soong-built modules used to automatically
include all the subdirectories of the root package, and also
the package of the entrypoint script. Soong is being updated
to remove that, to match bazel's behavior and soong's behavior
with embedded_launcher: true, so update these scripts to work
with the new behavior.

Bug: 245583294
Test: m pdl_python_generator_test pdl_python_parser_test && /ssd/aosp-master/out/host/linux-x86/nativetest64/pdl_python_generator_test/pdl_python_generator_test && /ssd/aosp-master/out/host/linux-x86/nativetest64/pdl_python_parser_test/pdl_python_parser_test
(cherry picked from https://android-review.googlesource.com/q/commit:3afdc71985d12471081c2b48d57cbd78e04e459c)
Merged-In: I6b64c0ed77e41348992cf266ce601977ac754f20
Change-Id: I6b64c0ed77e41348992cf266ce601977ac754f20