Improve interoperability with form2request and Scrapy Cloud support

[Gallaecio]

Regarding naming:

• _form2request is from an earlier draft where form2request was actually used as a dependency, but I ended up going for a simpler approach, where both libraries have APIs that make it easy for them to work together, while keeping them independent. But I could not come up with a better name for the private module, so I kept that. Glad to change it.
• I don’t like build_submission that much either, but I could not think of something better for a function that “finds a form of the given type, maps input data to form fields by field type, and finds the submission button”.

[Gallaecio]

I am not sure if the recommended default approach should be click=submit_button or click=submit_button or False, i.e. if Formasaurus cannot find a button, should we let form2request click the default submit button if it finds one, or force form2request not to click any submit button?

I suspect Formasaurus does not actively discard submit buttons, i.e. if there is one it will return it, in which case this would be a non-issue. If it turns out to be an actual issue, we can always update this line of the docs accordingly.