Implement Check_Collection to separate concerns for check filtering from Check_Repository

[felixarntz]

This PR introduces a Check_Collection interface, primarily with the purpose of providing enhanced filtering of checks to run, while decoupling that from the Check_Repository responsibilities.

The Check_Repository was introduced as the central place to register checks, and while a limited flag based selection mechanism makes sense in the scope of that class, the parameter for filtering down to specific checks wasn't really well thought through, which becomes more obvious when considering #308:

• We shouldn't just add more and more parameters to Check_Repository::get_checks() to allow further ways to filter.
• At the same time, we should arbitrarily implement filtering partially in Check_Repository and partially elsewhere.

The Check_Collection approaches relies on sort of a "builder" pattern where Check_Repository::get_checks() returns a collection that allows further filtering and largely can be used like an array. A to_array() method and a to_map() method are present to return the under-the-hood array representation. Internally, the checks are stored in the collection with their slugs to provide the necessary filtering and lookup capabilities.

[felixarntz]

It's worth noting that this PR also fixes a bug: Before this, Check_Repository::get_checks() would return a map ($slug => $check) when no $check_slugs were passed, but an indexed array of $check objects when $check_slugs were passed. This was simply due to an oversight. It can be seen especially in the tests updated in this PR: The one test where a slug was passed to Check_Repository::get_checks() is the only scenario which now uses to_array(), and it is the only place where an indexed array was expected before in the test - which didn't make sense and was probably implemented like that simply to make the test pass.

Now, it returns the collection and then the caller can decide whether they need the map or array format, via to_map() / to_array().

[mukeshpanchal27]

Thank you, @felixarntz, for the PR. It looks quite solid from my perspective. I've left some feedback to seek clarification on certain points.

[mukeshpanchal27]

Thanks @felixarntz for the changes. LGTM from my end.