fix(invariant): honor targetContract setting, don't update targets if any

[grandizzy]

Motivation

Closes #5625
slightly different approach than proposed in #6200

Solution

• add is_updatable flag in FuzzRunIdentifiedContracts, false if there's any contract marked as targetContract
• record new contracts created only if is_updatable
• tests for both cases, some code cleanup

follow up: document targetInterfaces usage

[DaniPopes]

This looks fine, but I'd like some more eyes on it @mattsse @klkvr
Pending also conflicts

[Evalir]

This looks good re: code, but would like @mds1 to test this out

[mattsse]

lgtm