plugins: rename SCPLuginFileType to SCEveFileType

plugins: rename SCPLuginFileType to SCEveFileType

With internal code using the plugin API to register an Eve
filetype, the name plugin no longer makes sense. This is
part of my idea that internal plugins aren't plugins at all,
and the plugin interface should be an abstraction over
internal APIs.

Along that idea, this is the start of a refactor of the plugin
file types to be internal, where the plugin API is just an external
interface to that internal API.