fix: optimise WIP pacts by using branch/tag heads

[bethesque]

PACT-1845

[bethesque]

A consumer version may have tags associated with it. Tags were what we used before we had branches and environments. Many customers still use tags to represent their branches. For these customers the "latest pact for a tag" generally equates to "the latest pact for a branch".

When calculating the set of pacts that should be returned for verification, there are two subsets.

1. Those specified by the consumer version selectors eg. { branch: "main", latest: true}, { tag: "prod", latest: true} and
2. the "work in progress pacts", where we attempt to return what we think are the pacts that are currently being worked on.

For both subsets of pacts, for customers who use tags, we identify the latest pact for each consumer version tag.

The exact logic for "latest pact for a tag" though, has some subtleties around it, and has changed over time. This is because we discovered problems with the original implementation.

Imagine "Some Consumer" which has published pacts for 3 application versions, which it has tagged with the branch name "main". For the first version, it publishes pacts with Providers 1 and 2. By the time it publishes v2, things have changed, and it no longer requires the integration with Provider 2, so it only publishes the pact for Provider 1.

Imagine we are fetching the pacts for verification. The original Pact Broker logic used to return the pact for Provider 2, even though that integration was no longer necessary. This was discovered to cause problems when verifying pacts, because that old pact would stick around in the verifications, even though the consumer didn't need that provider any more, and if the provider changed its behaviour in a way that caused that old pact to fail, it was unnecessarily blocking that provider from changing.

The new logic only returns pacts that belong to the latest version for each tag, so if a pact for an integration stops being published, it will stop being verified.

This new logic had been applied to subset 1 of the 'pacts for verification' response - the pacts specified by the consumer version selectors. It hadn't been applied to subset 2 of the pacts - the WIP pacts.

This PR updates the WIP pacts logic to use the "new" approach for finding the "latest pact for each tag". The upside of making this change is that as well as correcting the logic, it is a much more efficient query, and should help fix some performance issues we've been having.