Detect when previous merge is already in the load order

[focustense]

There is currently nothing to prevent loading a previous merge and making that a dependency of the new merge - probably by accident.

Plugins that are actually merges previously created by the app should either be disabled in the load order screen, or simply ignored. This is similar to how a Bashed Patch can't be imported as its own dependency.

[focustense]

I'm going to add one bit to the scope here, which is labeling and preventing the inclusion of plugins with missing masters. A few users ran into this issue and it's very hard to provide clear and useful error messaging after the fact; it makes much more sense to provide a UI like xEdit or Mod Organizer where those plugins are disabled or crossed out.

Although Mutagen is perfectly capable of reading plugins without the masters loaded, EasyNPC is very quickly going to try to actually locate those master records, and fail every time that happens.

[focustense]

Fixed in 1a80c14.