Revert "Move compilation to a role."

This reverts commit 7a90cbdc5486148bf15e0a62992d02e929ebf0b3. It was too
specific to the plugin to be usable by `msg-compile`, which was updated to do
its own thing in 93ca4a2c7. So just go back to having the plugin do its own
thing. There is a bit of code duplication, but not much. I can certainly live
with it.