Plugins: Move discovery logic to plugin sources (#106911)

* move finder behaviour to source

* tidy

* undo go.mod changes

* fix comment

* tidy unsafe local source

pkg/plugins/ifaces.go

@@ -18,9 +18,12 @@ type Installer interface {
 }
 
 type PluginSource interface {
+	// PluginClass is the associated Class of plugin for this source
 	PluginClass(ctx context.Context) Class
-	PluginURIs(ctx context.Context) []string
+	// DefaultSignature is the (optional) default signature information for this source
 	DefaultSignature(ctx context.Context, pluginID string) (Signature, bool)
+	// Discover finds and returns plugin bundles from this source
+	Discover(ctx context.Context) ([]*FoundBundle, error)
 }
 
 type FileStore interface {