Auto-format whitespace between imports

As a follow-up to !1697 (merged), this MR updates our Gitaly-specific Go formatter bracefmt to also manage whitespace in import blocks.

Because this is no longer about braces, we rename the formatter to gitalyfmt.

The current implementation is very crude: it expects at most one block of stdlib imports, an empty line, and then at most one block of non-stdlib imports. But if we can live with this it's one less thing to write review comments about.