allow providing prefixes declarations separate from list of prohibited within-namespace equivalents

Prefix declarations are needed for the input TSV. Right now inferred equivalents within these namespaces are also prohibited. @matentzn would like to be able to enable/disable that feature more selectively.

Start by locating the input TSV prefix-declaration handling and the logic that prohibits inferred equivalents within namespaces. Determine how those paths are currently coupled; the work is done when prefix declarations can be enabled independently from the prohibition, with behavior covered for both settings.

Written by the indexing model from the issue text.