guix package: Build up the transaction incrementally.

guix package: Build up the transaction incrementally.

* guix/scripts/package.scm (upgraded-manifest-entry): Rename to...
(transaction-upgrade-entry): ... this.  Add 'transaction' parameter and
return a transaction.
(options->installable): Likewise.
[to-upgrade]: Rename to...
[upgraded]: ... this, and change to be a transaction.  Return a
transaction.
(options->removable): Likewise.
(process-actions): Adjust accordingly.
* tests/packages.scm ("transaction-upgrade-entry, zero upgrades")
("transaction-upgrade-entry, one upgrade"): New tests.