When writing out the UCL file containing a package's metadata, metadata
values consisting of key-value pairs were not sorted. For instance, in
the certctl package we could have either

"deps": {
	"FreeBSD-openssl": {
		...
	},
	"FreeBSD-caroot": {
		...
	},
}

or

"deps": {
	"FreeBSD-caroot": {
		...
	},
	"FreeBSD-openssl": {
		...
	},
}

This breaks reproducibility.

Use the undocumented third parameter to libucl's to_format() to request
recursive sorting of keys.