Fix lookup logic for linked namespaces

When looking for already loaded libraries include
linked namespaces to the search, but check if
the library is accessible from the main namespace.

Bug: http://b/36008422
Bug: http://b/35417197
Bug: http://b/34052337
Bug: http://b/36660652
Bug: https://issuetracker.google.com/36636090
Test: run bionic-unit-tests --gtest_filter=dl*:Dl*
Change-Id: Ic7c1d48114da3ca5dc6512ef03f595dd17b6ed17