Having to replace - with _ (and vice versa) makes the slugs less greppable
and thus constitutes a contributor roadblock.
Result of running this repeatedly up until reaching a fixpoint:
find compiler/rustc_error_messages/locales/en-US/ -type f -exec sed -i 's/\(.+\)-\(.*\)=/\1_\2=/' {} \;
Plus some fixes to update usages of slugs leading with -.
