Add syntax for package and library (#792)

Doesn't add much logic, only takes advantage of parser structure for the ordering enforcement.

Note import_nonexistent tests should probably fail, but writing import tests needs a chain of functionality, and I figured I'd just start adding some to validate the syntax (not adding existent imports because that'd require multi-file structure).

executable_semantics/ast/ast.h

@@ -0,0 +1,30 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_AST_AST_H_
+#define EXECUTABLE_SEMANTICS_AST_AST_H_
+
+#include <list>
+
+#include "executable_semantics/ast/declaration.h"
+#include "executable_semantics/ast/library_name.h"
+#include "executable_semantics/common/ptr.h"
+
+namespace Carbon {
+
+// A Carbon file's AST.
+struct AST {
+  // The package directive's library.
+  LibraryName package;
+  // The package directive's API or impl state.
+  bool is_api;
+  // Import directives.
+  std::vector<LibraryName> imports;
+  // The file's ordered declarations.
+  std::list<Ptr<const Declaration>> declarations;
+};
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_AST_AST_H_