Remove tf_http_archive external Python deps.
Migrate TF Python deps from being defined as tf_http_archives in workspace2.bzl to being in requirements.in. This consolidates where Python dependencies are defined, allows them to be more easily updated and also allows any version updates to be easily transferred to requirement.txt locked versions using the Hermetic Python requirements updater. Remove any downstream references to the deleted tf_http_archive repositories and if relevant replace them with @pypi_*name*//:pkg.