AAPT2: Fail compiling when private symbols are referenced

Also moved some XML specific stuff into its own directory,
and refactored ReferenceLinker a bit.

Change-Id: I912247a82023c1bbf72dc191fbdaf62858cbec0c
diff --git a/tools/aapt2/xml/XmlUtil.cpp b/tools/aapt2/xml/XmlUtil.cpp
new file mode 100644
index 0000000..ab9f544
--- /dev/null
+++ b/tools/aapt2/xml/XmlUtil.cpp
@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "util/Maybe.h"
+#include "util/Util.h"
+#include "xml/XmlUtil.h"
+
+#include <string>
+
+namespace aapt {
+namespace xml {
+
+Maybe<ExtractedPackage> extractPackageFromNamespace(const std::u16string& namespaceUri) {
+    if (util::stringStartsWith<char16_t>(namespaceUri, kSchemaPublicPrefix)) {
+        StringPiece16 schemaPrefix = kSchemaPublicPrefix;
+        StringPiece16 package = namespaceUri;
+        package = package.substr(schemaPrefix.size(), package.size() - schemaPrefix.size());
+        if (package.empty()) {
+            return {};
+        }
+        return ExtractedPackage{ package.toString(), false /* isPrivate */ };
+
+    } else if (util::stringStartsWith<char16_t>(namespaceUri, kSchemaPrivatePrefix)) {
+        StringPiece16 schemaPrefix = kSchemaPrivatePrefix;
+        StringPiece16 package = namespaceUri;
+        package = package.substr(schemaPrefix.size(), package.size() - schemaPrefix.size());
+        if (package.empty()) {
+            return {};
+        }
+        return ExtractedPackage{ package.toString(), true /* isPrivate */ };
+
+    } else if (namespaceUri == kSchemaAuto) {
+        return ExtractedPackage{ std::u16string(), true /* isPrivate */ };
+    }
+    return {};
+}
+
+void transformReferenceFromNamespace(IPackageDeclStack* declStack,
+                                     const StringPiece16& localPackage, Reference* inRef) {
+    if (inRef->name) {
+        if (Maybe<ExtractedPackage> transformedPackage =
+                   declStack->transformPackageAlias(inRef->name.value().package, localPackage)) {
+            ExtractedPackage& extractedPackage = transformedPackage.value();
+            inRef->name.value().package = std::move(extractedPackage.package);
+
+            // If the reference was already private (with a * prefix) and the namespace is public,
+            // we keep the reference private.
+            inRef->privateReference |= extractedPackage.privateNamespace;
+        }
+    }
+}
+
+} // namespace xml
+} // namespace aapt