if we look up a feature, and of the known providers it is only supported by the default provider, don't go scanning the drive to see if there's a plugin to take precedence.

this optimizes usage of built-in features, so that if there's no plugin present for, say, "sha1", the qca app won't hit the disk drive every single time it is invoked just to see if a plugin supporting "sha1" has appeared.

the drawback is that if you want a plugin to take precedence over the default provider, then it needs to be installed before the application is run (or, the app can use QCA::scanForPlugins during runtime, to force the plugin to get noticed).