Anchor GatherDir's exclude_filename rules to avoid erroneous matches

The filenames specified for exclusion in GatherDir's `exclude_filename`
rule were added to the regex anchored with `\b$filename\b`, but that
is incorrect because filenames can have word boundaries. Since we're
matching against the whole path name, relative to the root, it
should suffice to anchor with `^` and `$` instead.

Fixes [RT#81492](https://rt.cpan.org/Public/Bug/Display.html?id=81492) for Dist-Zilla: GatherDir uses exclude_filename as
    a regex, causing files to be excluded incorrectly