Unclear error message when extending a non-existing imported function

```
julia> module Foo
       import Base: f

       f(x) = x
       end
WARNING: Imported binding Base.f was undeclared at import time during import to Foo.
ERROR: invalid method definition in Foo: exported function Base.f does not exist
```

The error talks about "exported function `Base.f`" but `f` is not exported from Base.