Skip child modules when running RustFmtRange

[drdavella]

The RustFmtRange command should not attempt to reformat any child modules of the module currently being formatted.

Without this change, I get the following error when attempting to format a range:

rust.vim: was not able to parse rustfmt messages. Here is the raw output:

Error writing files: failed to resolve mod `foo`: /private/var/folders/lx/810r91ls7nz_mf24vhv_94w40000gp/T/vWfCEK6/dy
foo.rs does not exist
Error detected while processing function rustfmt#FormatRange[4]..<SNR>92_RunRustfmt:
line  104:
E776: No location list

While --skip-children is unstable, the --file-lines option itself is also unstable, so this seems like it shouldn't be an issue.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @chris-morgan (or someone else) soon.

Please see the contribution instructions for more information.