The -plugin-dir option lets the user specify custom search paths for
plugins. This overrides all other plugin search paths, and prevents the
auto-installation of plugins.
We also make sure that the availability of plugins is always checked
during init, even if -get-plugins=false or -plugin-dir is set.
Previously we had a "getProvider" function type used to implement plugin
fetching. Here we replace that with an interface type, initially with
just a "Get" function.
For now this just simplifies the interface by allowing the target
directory and protocol version to be members of the struct rather than
passed as arguments.
A later change will extend this interface to also include a method to
purge unused plugins, so that upgrading frequently doesn't leave behind
a trail of unused executable files.
