Declare included files via dependency message

[mischnic]

As described in this old issue, files that were included/inlined should be declared as a dependency message so that tools know to re-run PostHTML (and this plugin) once one of these dependencies changes.

https://github.com/posthtml/posthtml/blob/b453a8223a9d4da6fb291f82948d6eb7dc9b615a/docs/plugins/index.md#messages

Coming here from parcel-bundler/parcel#3218