Don't build sdist by default in maturin build command

[messense]

This replaces --no-sdist with --sdist option to only build sdist
when --sdist is specified.

Part of #796

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[messense]

@konstin I wonder if we should keep --no-sdist in maturin publish command?

[konstin]

good question, not sure tbh. i'd like poeple to publish sdists, on the other hand that's where the file-already-exists errors come from

[messense]

Yes, publish sdist by default only works well when publish for single OS and architecture only, otherwise file-already-exists errors are inevitable unless --skip-existing is specified.

I'm leaning towards keeping --no-sdist in publish, like we have --no-strip in publish but --strip in build.