Fix up .orig detection to be less trustful of (ambiguous) filenames.

Fix up .orig detection to be less trustful of (ambiguous) filenames.

We cannot simply match the filenaem because `foo_1.0.orig.tar.gz'
might be the `.orig.tar.gz' of foo version 1.0, or the `.tar.gz' of
foo version 1.0.orig.

We need either to consider the whole .dsc (this works since the
formats with no `.orig' or `.debian' or whatever are precisely ones
where there is only one file anyway), or know the version number.

Replace is_orig_file with two functions, one for each situation.

While we're here, do not barf if we find uncompressed tarballs.  (We
don't expect to find any but it would be nice if they worked...)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>