This migrates everything but the `mbe` and `proc_macro` modules. It also
contains a few cleanups and drive-by/accidental diagnostic improvements
which can be seen in the diff for the UI tests.
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 -.
