Conan package registry: the project-level search endpoint should not return instance-wide results
## :fire: Problem
The GitLab [conan package registry](https://docs.gitlab.com/ee/user/packages/conan_repository) supports the `$ conan search` command.
This command will hit the `/search` endpoint. This endpoint is expected to do a fuzzy search on package names.
The GitLab [conan package registry](https://docs.gitlab.com/ee/user/packages/conan_repository) is exposed at two levels:
1. project-level
1. instance-level
The issue we noticed is that the search endpoint for the project-level is returning packages from outside the target project.
This doesn't make sense as `$ conan search` will display packages that can't be pulled using that project-level endpoint (that's because the download endpoints will properly scope to the target project = they can't return packages _outside_ the project).
## :fire_engine: Solution
* Properly scope the conan packages search when the project level endpoint is used. Only packages of that project should be returned.
* Take this opportunity to better split the services and finders. Currently, it's one class for multiple scenarios (eg. finding a single package vs multiple packages on project-level endpoint vs instance-level endpoint).
## :warning: Caution
Since we're changing the behavior of an endpoint, this is a ~"breaking change".
issue