packages: 'package-grafts' trims native inputs.

packages: 'package-grafts' trims native inputs.

'package-grafts' returns a list of potentially applicable grafts, which
'cumulative-grafts' then narrows by looking at store item references and
determining the subset of the grafts that's actually applicable.

Until now, 'package-grafts' would traverse native inputs and would thus
return a large superset of the applicable grafts, since native inputs
are not in the reference graph by definition.  This patch fixes that by
having 'package-grafts' ignore entirely native inputs from the
dependency graph.

* guix/packages.scm (fold-bag-dependencies)[bag-direct-inputs*]: Add
special case for libc.
* guix/packages.scm (bag-grafts)[native-grafts, target-grafts]: Remove.
[grafts]: New procedure.
Use it.
* tests/packages.scm ("package-grafts, grafts of native inputs
ignored"): New test.