Fix python's lib folder search on virtualenv

[DjLegolas]

the current way to find it looks for a file that was changed after a refactor virtualenv had

[danyeaw]

Hi @DjLegolas, thanks so much for another contribution! Since there is no issue linked to this and your description is pretty brief, could you please explain more about what issue this is fixing and under which condition? It will help me with context while reviewing it. Thanks!

[DjLegolas]

@danyeaw yeah sure, I just forgot to add the relevant details.

Basically, the issue happens when compiling a debug build, including python support and using a venv.
For gvsbuild to build gobject-introspection with debug support, it searches for the libs folder that should contain python's debug lib (python3x_d.lib file).
The issue it that the way of getting the base path of the interpreter was changed. Currently, the path to the base is located inside a file pyenv.cfg in the root of the venv folder. Therefor, I added the support for it.
Nevertheless, as I'm not sure that we need to still support the older way, I decided to kept it, just in case.

This issue was partially reported in #1451, but it also reports another issue with the build. I suspect it's something related to the upstream, but wasn't able to figure out yet.

[danyeaw]

Hi @DjLegolas, got it, thanks for the explanation! I don't know of any recent changes to virtualenvs since the venv module was added in Python 3.3 with PEP 405. Looking for the pyenv.cfg file makes sense, I vote that you remove the legacy method.

[DjLegolas]

@danyeaw the change is not recent, and probably not related to the builtin venv module from python.
In the original virtualenv module, on versions before 20.0.0, the orig-prefix.txt was in use.
Therefor, when the python_find_libs_dir function was added back in 4583c94 (2017), the virtualenv module had only the orig-prefix.txt, but it seem it was never tested on the builtin venv module.

Looking for the pyenv.cfg file makes sense, I vote that you remove the legacy method.

I have removed the legacy part - will now only support pyvenv.cfg.

[danyeaw]

Thanks @DjLegolas, this is a great fix!