`fun` allows references of local functions from the outside

[jponge]

Spotted by @yloiseau in https://github.com/eclipse/golo-lang/pull/312#issuecomment-157657194

[jponge]

Given:

    module Other

    local function bar = |x| -> x

and:

    module Test

    local function foo = |x| -> x

    function main = |args| {
      let f = fun("foo", Test.module)
      println(f(42))

      let b = fun("bar", Other.module)
      println(b(42))
    }

then the following works when it should not:

    $ golo golo --files other.golo test.golo 
    42
    42