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.
We can filter the allowed versions and sort them before checking the
protocol version, that way we can just return the first one found
reducing network requests.
Get provider needs to be provided with the plugin protocol version,
because it can't be imported here directly.
The plugin url types and methods were confusing; replace them with a few
functions to format the urls.
Add discovery.GetProviders to fetch plugins from the relases site.
This is an early version, with no tests, that only (probably) fetches
plugins from the default location. The URLs are still subject to change,
and since there are no plugin releases, it doesn't work at all yet.
Martin Atkins