Requirements: Remove/Deprecate 'item' from relatesTo

[jpmckinney]

relatedItem implies relatesTo = 'item'. Similarly, relatedLots would imply relatesTo = 'lot' (though this code doesn't exist).

relatesTo should be reserved for 'tenderer' and 'bid' #110.

If relatedItem or relatedLots is set and relatesTo is also set, that's an incoherence data quality issue.

Requiring publishers to set relatesTo when relatedItem or relatedLots is set is unnecessary and creates an opportunity for inconsistency.

As part of this issue, we can also greatly simplify the description of relatedItem:

Where relatesTo = "item" this field must be populated with the id of the item in this tender section which the criterion relates to. Where relatesTo <> "item" this field should be omitted.

The description of relatedLots is much simpler:

The identifiers of the lots to which this criterion applies.

[jpmckinney]

I've now closed #110, so we now actually have the option to eliminate relatesTo entirely and instead add a boolean like relatesToTenderer (consistent with OCDS) or relatesToBidder (consistent with bid extension) or relatesToSubmitter (to "future proof" for OCDS 2.0 open-contracting/standard#906). That said, it might be best not to start introducing the new term, to avoid having 3 terms at the same time.

[yolile]

Note that the responseSource codelist also uses the code "tenderer", so if we want to be consistent with that field code, we should keep tenderer, I guess. (or change that code too)

[yolile]

As part of this, we could update the extension documentation that currently says

"The extension is designed to allow procuring entities or buyers to express criteria, relating to either items being procured or bidders themselves, as structured data."

, without mentioning lots.

[jpmckinney]

Only moldova uses relatesTo (similar to #190)

I've changed it to relatesToTenderer (matching OCDS).

I've mentioned lots consistent (#191 (comment)), simplified the description of relatedItem (#191 (comment)), and in general applied the style guide.

open-contracting-extensions/ocds_requirements_extension@54b202f