Harmonise orderly_artefact() with orderly_dependency etc

[richfitz]

This PR does part 1 of harmonising orderly_artefact() with orderly_resource and friends:

• the description argument, which other functions lack becomes optional in the schema and the function definition
• warn if orderly_dependency() is used where the description arg is unnamed

This latter change, once in the wild, will make it less disruptive to change the order of the arguments, so that we have

orderly_artefact <- function(files, description = NULL) ...

See #150 for context

[plietar]

Is there any part of Packit that uses the artefacts from the metadata to view packets?

[plietar]

How about assert_scalar_character(description, call = environment(), allow_null = TRUE) ?

Quick grep through gives me a handful of occurrences of this pattern.

[richfitz]

thanks, good idea

[M-Kusumgar]

LGTM!