add `identifier-bindig-symbol'; fix free-id-table for renames

The implement of `free-id-table' uses `identifier-binding'
to decide on a symbolic name as a key for each identifier,
but `identifier-binding' doesn't provide enough information
for local and top-level bindings. The new `identifier-binding-symbol'
function provides that information.

Closes PR 13911
change `find-relative-directories' to normalize completed paths

Avoids problems with clients (like the Scribble part of `raco setup')
that compare paths.
games docs: fix an absolute-path problem