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/BUILD

@@ -37,6 +37,7 @@ cc_library(
         "//common:check",
         "//common:ostream",
         "//common:string_helpers",
+        "//executable_semantics/ast",
         "//executable_semantics/ast:declaration",
         "//executable_semantics/ast:expression",
         "//executable_semantics/ast:paren_contents",