Add "dir" and "*.info" to the default :files spec

This relates to #561, which is concerned with getting `info` files into packages where they are available.

Since most packages do not have `dir` or `*.info` files, this required changing the behaviour when a wildcard entry in `:file` expands to an empty file list. Previously, an error would be raised in this case; now, an error is raised only when the *entire* `:files` spec expands to an empty file list.

The slight danger, then, is that certain build failures due to upstream reorganisation of files might be less obvious.

Comments welcomed!