templater: factor out function to create mapping dict for nested evaluation overlaymap() is the hook point to drop mapping items conflicting with the default keywords which have to be re-evaluated with new 'ctx' resource.

templater: introduce resourcemapper class A couple more functions will be added later to work around nested mapping bugs such as the issue 5612.

log: do no expect templateresources() returning a dict The resources dict will be replaced with new resource mapper object, which won't implement __getitem__(key). Share the whole resources object with _graphnodeformater() to make porting easier.