Get the correct Python ABI tag of selected interpreter

[linw1995]

Pull Request Check List

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

resolve #378

get_abbr_impl is copied from https://github.com/pypa/packaging/blob/14ad3520dc866295963c677c6813646b8a59a6a6/packaging/tags.py#L801-L811

[frostming]

We could start to collect the in-process scripts in their own folder. Can you help do it in this PR?

pdm/models/in_process
- __init__.py    # Exports all functions here
- pep508.py
- get_abi_tag.py

[linw1995]

Very happy to help to do it.

[frostming]

This way we can directly use the __file__ variable to get rid of importlib.import

[codecov-io]

Codecov Report

Merging #379 (1e958f7) into master (db63172) will increase coverage by 0.48%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
+ Coverage   82.30%   82.79%   +0.48%     
==========================================
  Files          63       63              
  Lines        5183     5160      -23     
  Branches      933      926       -7     
==========================================
+ Hits         4266     4272       +6     
+ Misses        645      619      -26     
+ Partials      272      269       -3     

Flag	Coverage Δ
unittests	82.75% <85.71%> (+0.52%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pdm/utils.py	71.32% <ø> (+3.67%)	⬆️
pdm/models/builders.py	81.52% <50.00%> (ø)
pdm/models/pip_shims.py	90.19% <66.66%> (+8.94%)	⬆️
pdm/models/environment.py	85.78% <100.00%> (+0.54%)	⬆️
pdm/models/in_process/__init__.py	90.00% <100.00%> (ø)
pdm/cli/actions.py	74.48% <0.00%> (ø)
pdm/cli/utils.py	81.34% <0.00%> (+0.79%)	⬆️
pdm/formats/requirements.py	87.09% <0.00%> (+1.07%)	⬆️
... and 2 more

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[frostming]

Thank you for taking care of this