We ought to consolidate code in Wrangler for computing a URL given an absoluteURL and a RID

To install a ruleset, you have two options:

1. Provide the absolute (full) URL of where the source code is located, or
2. Provide an absolute URL of an already-installed ruleset (usually the one that is evaluating) and the RID of another ruleset whose source code is located in the same folder (see RFC-3986, section 4.2)

As Wrangler is written now, there are two places where we compute an absolute URL from a given URL and a RID. These are:

1. The rule install_ruleset_relatively here, which I view as the "official" place
2. The rule initialize_child_after_creation here, which should be changed in lines 410-411 to raise the same event but with different attributes