(PUP-2912) PMT Build symlink exception

Prior to this commit the build command would add symlinks to packages. This
can cause unexpected behavior when installed on a system.
This commit causes the build command to throw an error when it finds a symlink.
That should prevent new modules from being created with a symlink included.
I'm inclined to raise a special exception for this and catch it in forge/local_tarball ModuleRelease#unpack.