Extract downloaded file copying from operations.prepare

[chrahunt]

What's the problem this feature will solve?

For pip download and pip wheel, pip copies downloaded files to the output directory while it is doing preparation of requirements here.

Pulling this logic out of operations.prepare will allow us more flexibility to implement the new resolver without breaking pip download or pip wheel, because:

1. we will not be obligated to re-use RequirementPreparer in order to have files downloaded
2. we will not have to implement the same functionality in new objects collaborating to do the new dependency resolution

Describe the solution you'd like

Do what the code here is doing, except in pip._internal.commands.DownloadCommand and pip._internal.commands.WheelCommand.

Alternative Solutions

1. Keep the logic for downloads in RequirementPreparer - since RequirementPreparer is so tightly coupled to Resolver we are unlikely to use it for the new resolver, so we would need to worry about reimplementing this logic

Additional context

• Splitting RequirementPreparer #7049

[pradyunsg]

I don't think this would be a build logic task -- more around how pip handles downloads vs building distributions from those downloads.

(@chrahunt if you're around, we can probably chat on IRC right now)