Fix layout loading when locally serving Web UI with symlinked Elements

When locally serving Web UI with a symlinked checkout of Elements, some dynamically loaded layouts fail to load. This is mainly caused by the path resolution of the symlinks, resulting in malformed urls. Some elements rely on a strategy that uses the parent element's importPath, with a fallback on their own importPath (e.g. <nuxeo-document-layout>). This only works when the (direct) parent element is a Web UI element:

• Document layouts: <nuxeo-document-layout> is inside <nuxeo-document-form-layout> (and other similar Web UI elements).
• Drawer search forms: <nuxeo-search-form-layout> is inside <nuxeo-search-form>.

However, this is not always the case:

• Drawer search results: <nuxeo-search-results-layout> is inside <template is=dom-if> inside <nuxeo-results-view>.
• Picker's form and results layouts: both inside <nuxeo-results-view>, with the results inside a <template is=dom-if>.

Breaking the view layout is as easy as: In <nuxeo-document-view>, wrap the <nuxeo-document-layout> with a <template is=dom-if if="[[iEvaluateAsTrue]]"> and layout will 404.

This is already visible in the user management section: create and edit layouts are not working.

After the search layouts were moved from Web UI to Elements (WEBUI-173 & ELEMENTS-1311) this problem was highlighted due to the search results layouts failing to load.