Use shortest possible constant name for filtering completions

[vinistock]

Motivation

I noticed that completion suggestions using non-qualified constant names were getting poorly ranked in the list. The reason is because we were always using the fully qualified name as the filter text.

For example

module Foo
  module Bar
    class Baz
    end

    # Triggering completion here would compare `B` against `Foo::Bar::Baz`, which makes
    # it rank poorly. In reality, we should be comparing it to `Baz` since it's inside
    # the same namespace
    B
  end
end

Implementation

Used the same idea as our insertion text - deleting all parts of the current nesting to find the shortest possible name for the constant. The only catch in this case is that we only want to do that if the user is not explicitly typing the fully qualified name of the constant - in which case we do want the filter text to include the namespaces.

Automated Tests

Added a test showing the desired behaviour.

[andyw8]

This will need an update since HAS_TYPECHECKER was removed in #1042.

[vinistock]

Updated all the places that weren't using the stub in the completion test.