Add support for global index-urls

[heliocastro]

Inside restricted environments, pip can be configures to use unique global.index-url. Even added as extra index url, PyPI still present and would cause an blocked network request.

Used pip class to proper detect configured location.

[TG1999]

@heliocastro thanks ++, please add some tests and also check https://github.com/nexB/python-inspector?tab=readme-ov-file#testing, additionally if there are any formatting errors please run make valid

[pombredanne]

@heliocastro I am closing this in favor of https://github.com/aboutcode-org/python-inspector/issues/180 ... I pinged there too.