x/tools/cmd/goimports: move custom group imports set with "-local" flag to bottom of imports with no spaces.

[effinsky]

Imports mal-grouped:

Current behavior of running goimports -local 'github.com/shi**' is:

Desired behavior is:

This is from neovim with vim-go. I don't think it's a config problem. I have run the cmd in the shell on the file as well, with same results.

Thanks!

[dr2chase]

@heschi @bradfitz