pack: Import (guix store database) only when '--localstatedir' is passed.

pack: Import (guix store database) only when '--localstatedir' is passed.

This is another way to address <https://bugs.gnu.org/32184>, which was
previously addressed in commit 19c924af4f3726688ca155a905ebf1cb9acdfca2.

* gnu/build/install.scm (register-closure): Move to...
* gnu/build/vm.scm (register-closure): ... here.  New procedure.
* guix/scripts/pack.scm (self-contained-tarball)[build]: Remove
now unneeded 'with-extensions' form and custom (guix config) module.
* tests/guix-pack.sh: Revert the strategy from
commit 19c924af4f3726688ca155a905ebf1cb9acdfca2.
* tests/pack.scm ("self-contained-tarball"): Likewise.