ed1a674c ("keg: allow selective linking at the file level") had a
regression which caused the link step to skip files with the same name
as a directory in the user's CWD.
Keg#link_dir yields a relative pathname, so assumptions cannot be made
about the nature of that pathname in the block.
Instead, introduce ":skip_dir" and ":skip_file" to replace the existing
":skip" directive. This way, we won't skip things marked ":skip_dir"
when linking a file, and vice-versa.
FixesHomebrew/homebrew#10860.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>
5b1957f13a