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/BUILD

@@ -4,6 +4,15 @@
 
 package(default_visibility = ["//executable_semantics:__subpackages__"])
 
+cc_library(
+    name = "ast",
+    hdrs = ["ast.h"],
+    deps = [
+        ":declaration",
+        ":library_name",
+    ],
+)
+
 cc_library(
     name = "class_definition",
     hdrs = ["class_definition.h"],
@@ -19,7 +28,6 @@ cc_library(
     name = "declaration",
     srcs = ["declaration.cpp"],
     hdrs = [
-        "abstract_syntax_tree.h",
         "declaration.h",
     ],
     deps = [
@@ -82,6 +90,11 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "library_name",
+    hdrs = ["library_name.h"],
+)
+
 cc_library(
     name = "paren_contents",
     hdrs = ["paren_contents.h"],

executable_semantics/ast/abstract_syntax_tree.h

@@ -1,17 +0,0 @@
-// 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_ABSTRACT_SYNTAX_TREE_H_
-#define EXECUTABLE_SEMANTICS_AST_ABSTRACT_SYNTAX_TREE_H_
-
-#include <variant>
-
-#include "executable_semantics/ast/declaration.h"
-#include "executable_semantics/common/ptr.h"
-
-namespace Carbon {
-using AST = std::list<Ptr<const Declaration>>;
-}
-
-#endif  // EXECUTABLE_SEMANTICS_AST_ABSTRACT_SYNTAX_TREE_H_

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_

executable_semantics/ast/library_name.h

@@ -0,0 +1,25 @@
+// 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_LIBRARY_NAME_H_
+#define EXECUTABLE_SEMANTICS_AST_LIBRARY_NAME_H_
+
+#include <string>
+
+namespace Carbon {
+
+// Identifies a particular library. For example, "Geometry//Objects/FourSides"
+// will have package="Geometry" and path="Objects/FourSides".
+struct LibraryName {
+  // The library's package.
+  std::string package;
+
+  // The package-relative path of the library. This defaults to the empty
+  // string.
+  std::string path;
+};
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_AST_LIBRARY_NAME_H_