Migrate from standard plugin package to Hashicorp go-plugin

[spitfire55]

Go's built-in plugin package, part of the standard library as of Go 1.8, is very finicky with dependency versions across plugins. If a plugin is built using Go Modules, and two plugins use the same package with different versions, then the build process gets angry. This will naturally create a lot of headaches.

Hashicorp's go-plugin package uses an RPC model instead of shared object, which should (hopefully) fix this problem. Migrating is going to be a pain in the ass, but its better to do it now then later.