Move issuable resolver logic to finders

What does this MR do?

This moves re-usable resolution logic out of resolvers and into finders (or at least re-usable service-like objects).

The rationale for this is that resolvers are not meant to be composable and re-usable. They are both unergonomic (needing specific parameters to instantiate) and error prone to use. This aims at making code simpler, more consistent and less verbose, with the bonus of fixing some potential N+1 performance issues.

Related to #217217 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team