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.h b/tools/aapt2/xml/XmlUtil.h
new file mode 100644
index 0000000..98e5520
--- /dev/null
+++ b/tools/aapt2/xml/XmlUtil.h
@@ -0,0 +1,85 @@
+/*
+ * 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.
+ */
+
+#ifndef AAPT_XML_XMLUTIL_H
+#define AAPT_XML_XMLUTIL_H
+
+#include "ResourceValues.h"
+#include "util/Maybe.h"
+
+#include <string>
+
+namespace aapt {
+namespace xml {
+
+constexpr const char16_t* kSchemaAuto = u"http://schemas.android.com/apk/res-auto";
+constexpr const char16_t* kSchemaPublicPrefix = u"http://schemas.android.com/apk/res/";
+constexpr const char16_t* kSchemaPrivatePrefix = u"http://schemas.android.com/apk/prv/res/";
+constexpr const char16_t* kSchemaAndroid = u"http://schemas.android.com/apk/res/android";
+
+/**
+ * Result of extracting a package name from a namespace URI declaration.
+ */
+struct ExtractedPackage {
+    /**
+     * The name of the package. This can be the empty string, which means that the package
+     * should be assumed to be the package being compiled.
+     */
+    std::u16string package;
+
+    /**
+     * True if the package's private namespace was declared. This means that private resources
+     * are made visible.
+     */
+    bool privateNamespace;
+};
+
+/**
+ * Returns an ExtractedPackage struct if the namespace URI is of the form:
+ * http://schemas.android.com/apk/res/<package> or
+ * http://schemas.android.com/apk/prv/res/<package>
+ *
+ * Special case: if namespaceUri is http://schemas.android.com/apk/res-auto,
+ * returns an empty package name.
+ */
+Maybe<ExtractedPackage> extractPackageFromNamespace(const std::u16string& namespaceUri);
+
+/**
+ * Interface representing a stack of XML namespace declarations. When looking up the package
+ * for a namespace prefix, the stack is checked from top to bottom.
+ */
+struct IPackageDeclStack {
+    virtual ~IPackageDeclStack() = default;
+
+    /**
+     * Returns an ExtractedPackage struct if the alias given corresponds with a package declaration.
+     */
+    virtual Maybe<ExtractedPackage> transformPackageAlias(
+            const StringPiece16& alias, const StringPiece16& localPackage) const = 0;
+};
+
+/**
+ * Helper function for transforming the original Reference inRef to a fully qualified reference
+ * via the IPackageDeclStack. This will also mark the Reference as private if the namespace of
+ * the package declaration was private.
+ */
+void transformReferenceFromNamespace(IPackageDeclStack* declStack,
+                                     const StringPiece16& localPackage, Reference* inRef);
+
+} // namespace xml
+} // namespace aapt
+
+#endif /* AAPT_XML_XMLUTIL_H */