Fix enum and type resolution

[Mingun]

• Fixes https://github.com/kaitai-io/kaitai_struct/issues/786
• Fixes https://github.com/kaitai-io/kaitai_struct/issues/1028
• It seems that fixes unreported problem when opaque types are enabled and during enum resolution type cannot be found, it is threated as opaque and then enum is assume to existing in it. I think, that this is not desired behavior
• Unifies type and enum resolution between type / enum keys and expression language
• Changes error messages slightly, tests in kaitai_struct_tests should be updated. PR: https://github.com/kaitai-io/kaitai_struct_tests/pull/128