pmaports.find: show extra-repos/systemd hint (MR 2480)

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
[17:33:53] NOTE: The package 'systemd' exists in extra-repos/systemd, but systemd is currently disabled
[17:33:53] ERROR: Could not find aport for package: systemd

Fixes: issue 2398