Make groupPrefix() private

This could be made public in future. But I expect that we will want to
use this logic for QL models as well then we will want to move it into a
different file, which will be much easier if it's all private at the
moment.

Author: owen-mc

go/ql/lib/semmle/go/dataflow/ExternalFlow.qll

@@ -91,7 +91,7 @@ private import internal.FlowSummaryImpl::Private::External
 private import codeql.mad.ModelValidation as SharedModelVal
 
 /** Gets the prefix for a group of packages. */
-string groupPrefix() { result = "group:" }
+private string groupPrefix() { result = "group:" }
 
 /**
  * Gets a package represented by `packageOrGroup`.