Move libassistant DEPS from src_internal to src

This change moves libassistant-related dependencies from src_internal to src as a part of chromium browser on git submodule migration.

Bug: 1409738
Change-Id: I297ed3f88c15379c0962667e3354b7668fb09cd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4338029


Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Cr-Commit-Position: refs/heads/main@{#1118449}