Support aliases for convert --from-index option

[analog-cbarber]

Support the ability to use aliases for the pypi index URL, e.g.:

whl2conda convert --from-index myindex mypackage

The alias could be taken from standard locations by default, specifically:

• [x] ~/.pypirc
• [ ] pyproject.toml (should these depend on what build backend is registered?)
  • [ ] tool.poetry.source entries
  • [ ] tool.pdm.source entries
  • [ ] hatch publish.index.repos.private entries
  • [x] a new tool.whl2conda config option

Since this type of conversion may occur outside a project, the pyproject.toml options are less useful. Technically the ~/.pypirc file is only for uploads, so you might not have an entry for a private index that you only pull from.

Another option would be to add a user settings file for whl2conda that could contain these alias definitions.

[analog-cbarber]

Maybe just support the ~/.pypirc file to start with