Use absolute Nunjucks import path

The appHeader component macro currently imports the appSiteSearch macro
using a relative URL. This prevents customisation of the appSiteSearch
macro using the Nunjucks import prioritisation logic. Bringing this
import inline with other import syntax across the plugin fixes this
issue.

For example, a consuming 11ty app wants to override the appSiteSearch macro,
so can now create a "_includes/components/site-search/macro.njk" template
to do that.

components/header/template.njk

@@ -1,4 +1,4 @@
-{%- from "../site-search/macro.njk" import appSiteSearch -%}
+{%- from "components/site-search/macro.njk" import appSiteSearch -%}
 {%- if options.navigation -%}
   {%- set headerClasses = " govuk-header--full-width-border" -%}
 {% elif layout == "product" or layout == "collection" %}