[Refactor] Use setup.py from root dir.

[ailzhang]

This is a copy of #2531 to merge into master.

A few things included in this PR:

• Move setup.py up to root dir so that we can run python setup.py install/develop.
• Remove dependency of taichi_core in build process. We used to call
  ti.core.get_runtime_dir()/get_version_string()/libdevice_path() from
  C++, but these information are already available in build process.
• Simplify python/build.py to call setup.py in the root. Tested
  build/test commands still work as expected.
• Update github workflows to use python setup.py install in linux/mac instead of
  setting env TAICHI_REPO_DIR.
• Developers used to customize taichi_core.so build using args passed
  into cmake. Under this new mechanism they can put everything inside
  TAICHI_CMAKE_ARGS and run setup.py using: TAICHI_CMAKE_ARGS="-DXXXX -DXXXX" python setup.py ..
• Move tests/conftest.py into tests/python folder. I think it was a
  bug that we skipped it when we copied tests folder when building wheels.

To explain how build works after this PR:
In short, c++ build is no longer a separate step. It's built as an
extension when you build taichi python package.

Developers can use

• python setup.py develop(or pip install -e .) in root dir to
  install taichi into their current python environment. Note in
  develop mode any local changes to python files take effect
  immediately. You only need to rerun this command when you change C++
  code.
• python setup.py install(or pip install .) to install taichi into
  your their current python environment. Note in install mode files
  are copied over to python's site-packages so your local changes won't
  take effect until you rerun the command.
• No more manipulation with TAICHI_REPO_DIR and PYTHONPATH.

A few known followups:

• I left windows job to remain using TAICHI_REPO_DIR since I don't
  have a windows machine. But it should be straightfoward to update if
  someone else can help.
• We can start removing TAICHI_REPO_DIR in the codebase after this PR since our
  development mode has changed.
• Update developer install documentation.
• I'm pretty sure the handling for windows in setup.py might have
  minor problems about relative paths but will leave it to debug in CI.
  It should be a simple fix.
• We probably need to reconsider if we want to include assets/examples
  and a few other .py files under python/taichi in the release.

ghstack-source-id: b82db21c4293c3281dbf3869cc91a927d234a4fa
Pull Request resolved: #2531

Related issue = #

[k-ye]

LGTM!!