install: Use (guix store database) instead of 'guix-register'.

* gnu/build/install.scm (register-closure): Add #:reset-timestamps? and
and #:schema; honor them.  Rewrite in terms of 'register-path'.
(populate-single-profile-directory): Add #:schema and honor it.  Make
/var/guix/profiles and /var/guix/gcroots.
* gnu/build/vm.scm (root-partition-initializer): Pass
 #:reset-timestamps? to 'register-closure'.
* gnu/system/vm.scm (not-config?): New procedure.
(guile-sqlite3&co): New variable.
(expression->derivation-in-linux-vm)[config]: New variable.
[builder]: Use 'with-extensions'.
(iso9660-image)[schema, config]: New variables.
Wrap build expression in 'with-extensions'; add 'sql-schema' call.
Remove GUIX from INPUTS.
(qemu-image)[schema, config]: New variables.
Wrap body in 'with-extensions'.
(system-docker-image)[not-config?]: Remove.
[config]: Use 'make-config.scm'.
[schema]: New variable.
[build]: Use 'with-extensions'.  Add call to 'sql-schema'.  Remove GUIX
from INPUTS.
* gnu/system/file-systems.scm (%store-prefix): Check whether
'%store-prefix' is defined.
* guix/scripts/pack.scm (self-contained-tarball)[not-config?]
[libgcrypt, schema]: New variables.
[build]: Wrap in 'with-extensions'.  Adjust imported module list to use
'make-config.scm' for (guix config).

gnu/build/vm.scm

@@ -354,6 +354,7 @@ SYSTEM-DIRECTORY is the name of the directory of the 'system' derivation."
       (for-each (lambda (closure)
                   (register-closure target
                                     (string-append "/xchg/" closure)
+                                    #:reset-timestamps? copy-closures?
                                     #:deduplicate? deduplicate?))
                 closures)
       (unless copy-closures?