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/tests.py

@@ -103,10 +103,9 @@ def _update_list(use_git_state):
     # Garbage collect unnecessary golden files.
     for golden in sorted(goldens):
         if golden not in tests:
-            print(
-                "Removing golden '%s.golden' because it has no test." % golden
-            )
-            os.unlink(os.path.join(_TESTDATA, golden))
+            filename = "%s.golden" % golden
+            print("Removing golden '%s' because it has no test." % filename)
+            os.unlink(os.path.join(_TESTDATA, filename))
 
 
 def _update_golden(test):