Windows plugins must be loaded from the same directory as the
heimdal.dll in order to ensure that "DLL Hell" is not revisited.
Since plugins cannot be grouped in a directory by "module" name
the plugins must have a name of the form:
plugin_<module>_<name>.dll
Construct the "plugin_<module>_" prefix string dynamically and
test for both the prefix and the ".dll" extension when building
the list of plugins to load.
Change-Id: I0ff7bf7287f8fb38061c226f0844d7d0f1fb59ec
ce3d2be1c7