Avoid noisy logging for packages that intentionally don't have any zkg.meta-specified scripts

[ckreibich]

@J-Gras hit a situation in which a package is content to only ship Zeek scripts with a contained plugin, not (also) via zkg.meta's script_dir. Leaving out that directive and not providing any Zeek scripts at the toplevel leads to:

WARNING  installing "/mypath": no __load__.zeek in implicit script_dir, skipped installing scripts

We have other such packages in the collection, too. Seems fine to just remove the warning, or demote it to debug level. Note that there's a related warning that would be good to confirm/check in this context, here.

[J-Gras]

Another option might be to keep the warning but allow explicitly unsetting the value (e.g. script_dir=<none>).