FIX: search for tkinter first in builtins

[matthew-brett]

Python compiled from Python.org source builds the tkinter module as a
built-in module, not an external module, as is the case for the packaged
builds of Debian etc:

>>> Tkinter.tkinter
<module '_tkinter' (built-in)>

This breaks the MPL algorithm for searching for tkinter symbols, which
loaded the external module .so file to get the symbols.

Try searching in the main program namespace for the tkinter symbols,
before looking for the extermal module .so file.

Thanks to github user ettaka for reporting : see
#7428

[NelleV]

Backported as 033c3f2

Thanks @matthew-brett !