riverlea - fix is_active for streams from external extensions

[ufundo]

Overview

Riverlea framework currently loads if the current theme is from the Riverlea extension.

This is a blocker for adding streams with other extensions.

Instead we should check whether _riverlea_core_ is in the search order for the current theme.

Before

• cant add a Riverlea stream in another extension

After

• can add a Riverlea stream in another extension

Comments

Example of an external stream, for testing purposes: https://lab.civicrm.org/extensions/deptfordcreek/-/tree/dev . Doesn't work properly before this patch.

[civibot]

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