If the user tries to build a package, but it can't be found: check if
systemd is disabled and the package is in extra-repos/systemd, display a
hint about it.
$ pmbootstrap build systemd[17:37:02] NOTE: The package 'systemd' exists in extra-repos/systemd, but systemd is currently disabled[17:37:02] ERROR: Could not find package 'systemd' in pmaports
