guix package: Introduce <manifest> and <manifest-entry> types.

* guix/scripts/package.scm (<manifest>, <manifest-entry>): New record
  types.
  (make-manifest, read-manifest, manifest->sexp, sexp->manifest,
  read-manifest, write-manifest, remove-manifest-entry, manifest-remove,
  manifest-installed?): New procedures.
  (profile-derivation): Take a manifest as the second parameter.  Use
  'manifest->sexp'.  Expect <manifest-entry> objects instead of
  "tuples".  Adjust callers accordingly.
  (search-path-environment-variables): Changes 'packages' parameter to
  'entries'.  Rename 'package-in-manifest->package' to
  'manifest-entry->package'; expect <manifest-entry> objects.
  (display-search-paths): Rename 'packages' to 'entries'.
  (options->installable): Change 'installed' to 'manifest'.  Have
  'canonicalize-deps' return name/path tuples instead of raw packages.
  Rename 'package->tuple' to 'package->manifest-entry'.  Use
  <manifest-entry> objects instead of tuples.
  (guix-package)[process-actions]: Likewise.  Rename 'packages' to
  'entries'.
  [process-query]: Use 'manifest-entries' instead of
  'manifest-packages'.