Add option to only get the "source" distribution of packages.

[tdruez]

python-inspector --spec lxml --json -

Returns 3 duplicated packages pkg:pypi/lxml@4.9.1 except for the download_url:

• lxml-4.9.1-cp38-cp38-manylinux_2_17_x86_64.manylinux2014_x86_64.manylinux_2_24_x86_64.whl
• lxml-4.9.1-cp38-cp38-manylinux_2_5_x86_64.manylinux1_x86_64.whl
• lxml-4.9.1.tar.gz

We should add a new option --prefer-source that would only return source distributions of packages.
This should be available in the API as well, not only in the CLI.

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Also, there's over 70 binaries distributions available at https://pypi.org/project/lxml/#files for this version of lxml 4.9.1
It seems arbitrary that the tool decided to only resolve for Python 38 on linux.

A better default would be to only return the source packages, and to force the user to be explicit about the --python-version and --operating-system.