(WIP) further cleaning

[sshiraiwa]

@justinlaughlin

Please take a look at this PR and see how I am cleaning up the setup.py

I started from imporve-install. I did a couple of things

1. added a custom backend, which seems necessary to handle -C "--build_options xxxx"
2. made routines for building libraries (metis xxx) in the separate file.
3. all routines needed for build is moved under _build_system
4. added build_globals.py to store build parameters, which can be changed based on user input
5. added build_consts.py to store constant parameters, which does not change once it set
6. used all to use "from xxx import *", instead of listing up each items

Status:
following command build MFEM, generate wrapper, and build wrapper.

 pip install .  -C"--verbose --build-options=--mfem-branch='master'" --no-cache-dir --no-binary mfem --verbose

However, I am having numpy 2.0 related issue.

[sshiraiwa]

@justinlaughlin
I think that this PR is ready to merge.
Please merge it to improve-install and undo "draft".
After combining this to PR278, we could merge PR278.