swig: move all swigged objects into m5.internal.swig package

SWIG-generated packages for params, enums, wrapped C++ SimObjects,
etc. were being generated directly into m5.internal, with some
funky code to import some of the declarations into m5.internal.params.
Moved all generated packages under m5.internal.swig and imported
things directly from there where necessary.  Cleaned up some
exposed loose ends.  Turns out m5.internal.params doesn't really seem
to be needed, so it's gone now.

Also renamed param_Foo packages that wrap both the C++ version of
Foo and FooParams to just Foo; now that they're under a separate
subpackage there's no ambiguity with the primary Python objects,
and no need to have the param_ prefix to support the funky
importing into internal.params.