Add an unnest_imports option to break apart nested import groups.

[goffrie]

This option breaks any nested imports containing a :: out of their import group to a new top-level use item, effectively desugaring the nested import feature added in Rust 1.25. This allows enforcing coding styles that don't allow the use of that feature, and also makes the merge_imports option useful in that case.

[calebcartwright]

Thank you for the PR. However, similar to #4553 I am going to close this because the implementation is going in the opposite direction of where we want to go to support various import merging/flattening strategies.

If you'd be interested in working on that target approach (and can certainly include this as one of the supported strategies) that'd be great!

rust-lang/style-team#140
#3362 (comment)

[goffrie]

Is this just a question of how the configuration options are exposed?

[calebcartwright]

Is this just a question of how the configuration options are exposed?

Basically. We want to have a single option that's an enum instead of myriad boolean options and have to deal with conflicting true values. IIRC what you'd proposed here would align to what folks were calling "shallow" (I may be recalling incorrectly though).

[calebcartwright]

Also if you're up for the different approach feel free to reopen and adjust text/commits accordingly, I went ahead and closed as I figured a new PR would be easier but entirely up to you 😄

[goffrie]

I can open a new PR.