fix imports within letin expressions

[kirchner]

Hi,

it seems like, that imports are not propagated outside of let in expressions. The change seems to fix this and also still works with my personal test project. I also added a test.

Best, Fabian

[mdgriffith]

Thank you and good catch! I appreciate the inclusion of a test as well!

I ended up removing the top level Elm.letIn in favor of the Elm.Let module which should be easier to use.

[kirchner]

Sounds good :)