Refactor operations.prepare.prepare_linked_requirement

[McSinyx]

Break operations.prepare.prepare_linked_requirement
into smaller (not small enough IMHO) methods.

This not meant to solve GH-7815 completely. I agree with #7815 (comment) that we should somehow put the populate_link in the same place for the logic to be easier to parse.

Rather, this is purely to make the method easier for me to read to proceed with more experiments on the downloading process.

As for the stylistic changes with the placement of the closing parentheses, I don't really have a strong opinion on them; although I'd be happier if they're not used for single-line argument list.

[pradyunsg]

Just talked with @McSinyx on a different communication channel.

This PR contains 2 changes currently -- the behavior change mentioned in the title and a refactoring change to address a "TODO: break this up" comment. He'll be breaking this PR into smaller PRs, for each of the separate changes.

[sbidoul]

@McSinyx to facilitate (and speed-up) review do not hesitate to split your PR in small commits, each one doing a small thing (such as factoring out a function, reformatting code, etc).

[McSinyx]

@pradyunsg, @sbidoul, I've just split the patch into multiple commits and hopefully it's nicer for review now.

[McSinyx]

Oh no I messed up rebasing stuff 😄

[pradyunsg]

I'd appreciate another pair of 👀 on this PR. ^>^

[McSinyx]

@sbidoul, I am sorry to ask for this on Sunday, but may I have your review on this PR?

[pradyunsg]

I'll go ahead and merge this, since this looks good to me. :)