EXCLUDE argument inconsistency

Hi,
some of the linters support a EXCLUDE argument where you can specify paths to not lint.

I made a test and it looks like the argument is interpreted differently in uncrustify and cpplint.
In my example, I want to exclude the file `external/cxxopts-2.1.2/include/cxxopts.hpp`.

I have the following CMake code

```
ament_cpplint(EXCLUDE ${_linter_excludes})
ament_uncrustify(
EXCLUDE ${_linter_excludes}
LANGUAGE C++
)
```

The problem is the following.
If I do `set(_linter_excludes external/cxxopts-2.1.2/include/cxxopts.hpp)` then the file is correctly excluded by `cpplint` but not by `uncrustify`.
On the other hand, if I do `set(_linter_excludes external)` the opposite happens (the file is correctly excluded by `uncrustify` but not by `cpplint`).

A dummy solution consists in having both file-level as well as folder-level exclusions, but this definitely seems a problem if I had to do more advanced exclusion logic.