Use a standard python site-packages directory

[chadrik]

The pxr module is located in $USD_PREFIX/lib/python but it would be great if it was stored using python's standard layout $USD_PREFIX/lib/python2.7/site-packages.

two reasons:

• it let's users run pip install PyOpenGL --prefix=$USD_PREFIX and it will just work.
• eventually we're going to need to deal with python3. yep, start embracing that idea because python2's EOL is coming :)

In general I've found that it just makes life easier to organize these things the way that python wants you to.

Side note: it would be good to add PyOpenGL to the list of USD's dependencies.

[jtran56]

Filed as internal issue #136417.