The 'file' and 'translations' attributes of the <include> element in the
org.eclipse.ui.editors.templates extension point previously only
accepted plug-in local file paths, resolved relative to the contributing
bundle via FileLocator.find(Bundle, IPath, Map).

This change extends ContributionTemplateStore.readIncludedTemplates to
also accept platform:/ URIs (e.g. platform:/plugin/<bundle-id>/<path>),
allowing a contributor to reference template files that live in another
plug-in. Bundle-relative paths continue to work unchanged; platform:/
URIs are resolved via FileLocator.find(URL).