src,module: add --preserve-symlinks command line flag

Add the `--preserve-symlinks` flag. This makes the changes added
in #5950 conditional. By default the old behavior is used. With
the flag set, symlinks are preserved, switching to the new
behavior. This should be considered to be a temporary solution
until we figure out how to solve the symlinked peer dependency
problem in a more general way that does not break everything
else.

Additional test cases are included.

PR-URL: https://github.com/nodejs/node/pull/6537


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>