fix: --download option

[mayeut]

Fixes #2120

Without those changes, downloads were only attempted for pinned versions.
When the --download option is used:

• Always trigger a download when using bundle, don't even look at what is embedded / updated wheels.
• Never trigger a download when using embed, only use the embedded version.
• When a pinned version is used, start by looking if it matches embedded / updated wheels and fallback to to trigger a download if not found.

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• ran the linter to address style issues (tox -e fix_lint)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[gaborbernat]

The fix now has been released via https://virtualenv.pypa.io/en/latest/changelog.html#v20-12-0-2021-12-31