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/test_list.bzl

@@ -61,6 +61,9 @@ TEST_LIST = [
     "if_nesting",
     "if_true",
     "ignored_parameter",
+    "import_late",
+    "import_nonexistent_library",
+    "import_nonexistent_package",
     "invalid_char",
     "match_any_int",
     "match_int",
@@ -68,6 +71,8 @@ TEST_LIST = [
     "match_placeholder",
     "next",
     "no_match",
+    "package_library",
+    "package_missing",
     "pattern_init",
     "pattern_variable_fail",
     "placeholder_variable",