<rdar://problem/15906684>

The way in which we were determining whether a python module had already been imported in the current session stopped working due to the IOHandler changes
As a result, importing in a new debug session a module which had been imported in a previous session did not work
This commit restores that functionality by checking for the module's presence in the session dictionary (which should be more correct anyway)



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201623 91177308-0d34-0410-b5e6-96231b3b80d8