fix collision protect errors by using localized install contents

Symlinks are followed to localize installed files which fixes
collision-protect errors when installing python packages using newer
python-exec that install into /usr/lib/python-exec which is a symlink to
/usr/lib64/python-exec on multilib amd64 systems.

This also reverts the previous workaround of disabling realpath that
caused issues when merging files under the symlinked directories. The
files would get removed every other install iteration because the
locations weren't matching (one would use the symlinked dir while the
other would use the real dir).
minor comment fixes and whitespace/wrapping consistency changes
use consistent variable name for merge engine objects
remove unused default_ldpath global variable

LDPATH is now discovered by scanning the files in /etc/env.d.