Make internal_backend plugins 1st class local pants plugins.

This moves wiring-up of the pants_internal backends to pants.ini
and make the (future) role of src/python/pants/bin:pants clear
and distinct from src/python/pants/bin:pants_local_binary.

It also fixes some broken docsite.json paths and adds some
TODOs to nuke 3rdparty/python/dev_only_requirements.txt.

Testing Done:
CI went green here: https://travis-ci.org/pantsbuild/pants/builds/36185854

# Publishing still works (publish_docs_new newly re-works)
$ ./build-support/bin/publish_docs.sh
$ ./build-support/bin/publish_docs_new.sh

# internal_backend tests still work
$ PANTS_DEV=1 ./pants goal test tests/python/internal_backend_test::
$ rm pants.pex && ./pants goal test tests/python/internal_backend_test::

# the local pants.pex contains the internal_backend
$ zipinfo pants.pex | grep internal_backend
-rw-r--r--  2.0 unx       55 b- defN 14-Sep-24 11:19 internal_backend/__init__.py
-rw-r--r--  2.0 unx        0 b- defN 14-Sep-23 14:29 internal_backend/sitegen/__init__.py
-rw-r--r--  2.0 unx      123 b- defN 14-Sep-24 11:18 internal_backend/sitegen/__init__.pyc
-rw-r--r--  2.0 unx      555 b- defN 14-Sep-23 14:29 internal_backend/sitegen/register.py
-rw-r--r--  2.0 unx      813 b- defN 14-Sep-24 11:18 internal_backend/sitegen/register.pyc
-rw-r--r--  2.0 unx       55 b- defN 14-Sep-24 11:19 internal_backend/sitegen/tasks/__init__.py
-rw-r--r--  2.0 unx     5749 b- defN 14-Sep-23 14:29 internal_backend/sitegen/tasks/sitegen.py
-rw-r--r--  2.0 unx     7291 b- defN 14-Sep-24 11:18 internal_backend/sitegen/tasks/sitegen.pyc

# The published sdists do not contain the internal_backend
$ ./pants setup_py //src/python/pants:pants-packaged //src/python/pants:test_infra
WARNING!  /home/jsirois/dev/3rdparty/jsirois-pants2/dist/pantsbuild.pants.47Hv89/src/pants/docs/conf.py is source but does not belong to a package!
WARNING!  /home/jsirois/dev/3rdparty/jsirois-pants2/dist/pantsbuild.pants.47Hv89/src/pants/docs/pagerank.py is source but does not belong to a package!
Running packager against /home/jsirois/dev/3rdparty/jsirois-pants2/dist/pantsbuild.pants-0.0.24
Writing /home/jsirois/dev/3rdparty/jsirois-pants2/dist/pantsbuild.pants-0.0.24.tar.gz
$ tar -tzf dist/pantsbuild.pants-0.0.24.tar.gz | grep internal_backend
$

$ ./pants setup_py //src/python/pants:test_infra
Running packager against /home/jsirois/dev/3rdparty/jsirois-pants2/dist/pantsbuild.pants.testinfra-0.0.24
Writing /home/jsirois/dev/3rdparty/jsirois-pants2/dist/pantsbuild.pants.testinfra-0.0.24.tar.gz
$ tar -tzf dist/pantsbuild.pants.testinfra-0.0.24.tar.gz | grep internal_backend
$

Bugs closed: 608

Reviewed at https://rbcommons.com/s/twitter/r/1073/
Sync of https://github.com/pantsbuild/pants/commit/5e8a6c9eee8656172a1a0a57957b6a2d94c58f46