(PUP-3191) Fix symlink checking

Prior to this commit we were trying to use Pathname.realpath to find where a
symlink points to. This throws a file not found when the symlink doesn't point
at anything valid in the filesystem.
This commit fixes that problem by only using File.readlink and not attempting
to find the symlinked file relative to the module unpacking directory.

Putting this up so folks know what the problem is. Will add ticket number and other info after lunch.