(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.
Another option to consider is *not* warning when unpacking a module with symlinks in it.

Yet another option involves warning only after the modules have been copied into their final place (`Forge::ModuleRelease#install`).