database: 'with-database' can now initialize new databases.

database: 'with-database' can now initialize new databases.

* nix/libstore/schema.sql: Rename to...
* guix/store/schema.sql: ... this.
* Makefile.am (nobase_dist_guilemodule_DATA): Add it.
* nix/local.mk (%D%/libstore/schema.sql.hh): Adjust accordingly.
* guix/store/database.scm (sql-schema): New variable.
(sqlite-exec, initialize-database, call-with-database): New procedures.
(with-database): Rewrite in terms of 'call-with-database'.
* tests/store-database.scm ("new database"): New test.
* guix/self.scm (compiled-guix)[*core-modules*]: Add 'schema.sql' to
 #:extra-files.