packages: Generalize package module search.

* gnu/packages.scm (%distro-root-directory): New variable.
  (%distro-module-directory): Remove.
  (package-files): Rename to...
  (scheme-files): ... this.  Return absolute file names, not stripped.
  (file-name->module-name): New procedure.
  (package-modules): Add 'directory' and 'sub-directory' parameters.
  Rewrite accordingly.
  (fold-packages): Adjust 'package-modules' call accordingly.