record some metadata for each package

[db48x]

I think it would be nice to be able to query package metadata without actually having to download the packages. This patch generates a metadata.sexp file for each package. For actually searching for packages it'd be nice to have a single file with all of the metadata, but I haven't gotten that far. Nor have I modified quicklisp-client to let users query this data. My goal for the moment is to allow Guix to install packages directly from the Quicklisp repository. I welcome your comments, suggestions, improvements, etc.

The code is probably not very beautiful, and system-file-magic is now doing two jobs at once. Better argument passing/parsing could help clean that up, or it could be changed to just output the metadata (and to stdout for easier access) so that the caller could do with it what they want. The existing callers could grab the list of systems, while write-metadata-sexps could save it to the file.