Fix some visibility issues with mocking modules and foreign functions:

[asomers]

• When mocking foreign functions, the 'foreign ""' block is not a
  module, and should not introduce an extra layer of visibility
  modifications.

• The visibility of the Expectation must always match the levels
  argument of Expectation::new().

• When mocking modules, the argument and return types of mocked
  functions must be superfied by one level. Otherwise code like this
  wouldn't compile:

  struct SuperT;
  
  #[automock]
  pub mod m {
      fn foo(x: super::SuperT) -> super::SuperT;
  }