Add a `--generate-only` option to the Complement launcher. (#16828)

Pulled out of #16803 since the drive-by cleanup was maybe not as
drive-by as I had hoped.

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Original commit schedule, with full messages:

<ol>
<li>

Add a --generate-only option 

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</ol>

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Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>