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/syntax/parse.h

@@ -8,7 +8,7 @@
 #include <string>
 #include <variant>
 
-#include "executable_semantics/ast/abstract_syntax_tree.h"
+#include "executable_semantics/ast/ast.h"
 
 namespace Carbon {