Don't copy (toplevel) DEBIAN directory to build dir

[John Lenton]

Currently if the user has a DEBIAN file, or symlink to a file, in the toplevel directory of a package, build will fail.

If it's a directory or a symlink to a directory, build will use that directory and copy its contents into the package as is (other than overwriting manifest, control, etc).

If it's a directory with e.g. manifest inside being a symlink to some other file, that other file is overwritten (if permissions allow).

Some or all of this is a bug. Way to avoid is to not copy DEBIAN, either by adding it to the exclude list (which would stop from copying anything called DEBIAN anywhere in the tree), or an explicit check for it (which would let the user still have other things called DEBIAN, for whatever reason).