services/horizon/interal/actions: Drop db2/core dependency in path finding actions.

[abuiles]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

• Drop db2/core dependency on path finding actions.
• Simplify tests for path finding, removing dependency on pre-built scenario.
• Add mock for paths.Finder

Why

We are in the process of removing Horizon's dependency on core's db. This PR removes such dependency on the path finding end-points and changes the tests to not depend on pre-built SQL scenarios.

This is part of the work to drop db2/core (#2643).

Known limitations

[abuiles]

instead of filling the database with order and build the graph for the test, I decided to go with a mock and test that the actions behave correctly with the given query parameters. We don't have to worry about the result since we already have unit test for orderbook.OrderBookGraph behavior.