RF: move EXT_LIBS to install names tests
RF+TST refactor tree_libs dict output

Change ``tree_libs`` output to use canonical (os.path.realpath) names
for both depending and depended libraries. The set of depending
libraries now becomes a dictionary with keys being the depending library
canonical names, and the values being the install name by which the
depending library refers to the depended library.

This means that we don't pick up the same library twice when there are
symlinks with different names pointing to the same library.
RF+TST: add strip_lib_dict function

With tests
RF+TST: refactor wheel_libs to new lib_dict

Refactor wheel_libs to use new lib_dict structure and new path stripping
function.
RF: rename strip_lib_dict to stripped_lib_dict

because it returns a new dictionary rather than modifying in-place.
DOC: update docstring for library analysis

Change references to install names - library dependencies is better put.
RF: rename wheel_libs lib_filt_func to filt_func

Inconsistent with ``tree_libs`` function signature.
RF: adapt delocate-listdeps to lib_dict structure

Use stripped_lib_dict to replicate more or less same output.
RF: refactor high-level routines to lib_dict mod

Refactor delocation routines to use the new lib_dict structure.

Refactor _copy_required and add lots of comments because I didn't
understand it.  I still don't properly.