feat: restructure directory

[Zhaoyilunnn]

What Changed

Restructure the directory. Moved quafu to the root directory from src.

.
|-- _skbuild
|   `-- linux-x86_64-3.10
|-- dist
|-- examples
|   `-- quantum_rl
|-- pyquafu.egg-info
|-- quafu
|   |-- __pycache__
|   |-- algorithms
|   |-- backends
|   |-- benchmark
|   |-- circuits
|   |-- dagcircuits
|   |-- elements
|   |-- exceptions
|   |-- pulses
|   |-- qfasm
|   |-- results
|   |-- simulators
|   |-- synthesis
|   |-- tasks
|   |-- users
|   |-- utils
|   `-- visualisation
|-- src
|   |-- qfvm
|   `-- qfvm_gpu
|-- tests
|   `-- quafu
`-- thirdparty
    `-- Eigen3

Why this change

The skbuild package has some bugs in enabling development mode. See scikit-build/scikit-build#1025. A workaround is to use python setup.py develop.

However, previously python modules and C++ source codes are all located under src directory.
And thus in previous setup.py, we specified the package_dir field as {'': 'src'}.
When using python setup.py develop it find the egg_base based on distribution's package_dir member (code pointer), normally {'': 'src'} will take effect and the egg_path will be set to src.

Unfortunately, skbuild has some other bugs, it overrides the distributions package_dir and {'': 'src'} is shown to be missing, causing the failure of setting the correct egg_path and always set the egg_path as ..

As a result, I think currently the best practice in developing python/c++ hybrid projects is to put python modules in the root directory and put c++ source code in the src directory.

I just checked that qiskit-aer follows the same practice.

Testing

python setup.py develop && pytest tests/
python setup.py install && pytest tests/
pip install . && pytest tests/

All these methods of installing pyquafu works fine