Update Import/Export ObjectBuilder for merge requests

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/manage/import/support/-/issues/7#note_836688337

This MR adds target_project_id to the select query in order to speed up project merge requests lookup when importing CI pipelines that reference existing merge requests.

Before this change the following SQL query was fired:

SELECT "merge_requests".* FROM "merge_requests" WHERE "merge_requests"."title" = $1 AND "merge_requests"."iid" = $2 LIMIT $3

As you can see it's not scoped to a specific project which can cause performance issues.

Query after the change:

SELECT "merge_requests".* FROM "merge_requests" WHERE "merge_requests"."title" = 'MergeRequest' AND "merge_requests"."target_project_id" = 119 LIMIT 1 

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