Clean up python import statements

[k-ye]

Explicit is better than implicit. -- PEP20

• Try to avoid import taichi as ti
  • Using public APIs may be an exception?
• No more from xx import *
  • Right now almost all the symbols (public API + implementation) are exposed to the top level package . This needs to be cleaned up to a point where only the public APIs are in taichi.
• Remove all the aliases of taichi.core.util.ti_core

  • taichi/python/taichi/lang/core.py

    Line 3 in c93a62f

    taichi_lang_core = util.ti_core

  • taichi/python/taichi/lang/ops.py

    Lines 13 to 14 in c93a62f

    	# TODO(#2223): Please stop abusing so many ti_cores
    	ti_core = taichi_lang_core

  • taichi/python/taichi/lang/__init__.py

    Line 13 in c93a62f

    core = taichi_lang_core

  • It's also confusing to define and load ti_core inside a module called util...
  • These aliases make it a bit hard to track whether the object is indeed initialized during import
• Move most of the imports to the top of the file
  • This is not a rule, though: https://stackoverflow.com/questions/1024049/is-it-pythonic-to-import-inside-functions
• Avoid cyclic import as much as possible. While this is allowed, it potentially introduces lots of initialization ordering problems. (e.g. https://stackoverflow.com/a/746067/12003165)
• Use isort or other similar tools to re-format the import statements. Additionally, add this to ti format
• Do not match the taichi functions based on string literals in transformer.py.
• ...