Plugins in `plugin-list` can now optionally specify an alias name, to
avoid naming collisions if plugins have the same repo name. For example,
specifying `author/common-plugin as author-common-plugin` will use
`author-common-plugin` as the package name in `generated.nix`.
Vim plugins were previously represented as strings by default,
necessitating a `knownPlugins` set. Backwards compatibility is kept
(strings are still accepted), so vam plugins should continue to work.
