Fix Lint check for duplicate external names

Lint was checking for duplicate external names by calling removeDups, which needs a comparison function that is passed to Data.List.sortBy.

But the comparison was not a valid ordering - it returned LT if one of the names was not external.

For example, the previous implementation won't find a duplicate in [M.x, y, M.x].

Instead, we filter out non-external names before looking for duplicates.