input/Open: use OpenLocalInputStream()

Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

src/decoder/DecoderThread.cxx

@@ -115,9 +115,7 @@ decoder_input_stream_open(DecoderControl &dc, Path path)
 
 	InputStream *is = OpenLocalInputStream(path, dc.mutex, dc.cond, error);
 	if (is == nullptr) {
-		if (error.IsDefined())
-			LogError(error);
-
+		LogError(error);
 		return nullptr;
 	}