fix nested enum not resolving

[zsylvia-verily]

In utils.rs, the find_enum_by_name and find_enum_by_name_in_message functions assume the enum is the last part of the full enum path and that the rest of the enum path is the package. It does not consider that the enum may be nested within another message. This results in the plugin searching for the enum .some.package.TestMessage.TestEnum in the "package" .some.package.TestMessage which is not actually a package, and thus it does not find it.

The unit test added reproduces this issue when tested against the previous version.

NOTE: I made the decision to only support one layer of nested enum rather than infinitely nested messages and enums to keep it simpler.

Issue: #36

[rkrishnan2012]

This fixes #38

[rholshausen]

0.3.7 released