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/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_