Make //mojo/public/python/BUILD.gn flexible wrt location in client repo

This CL adds the mojo_sdk config to //mojo/public/python:system and
//mojo/public/python:system_impl so that the (generated and manually written)
C++ source files built via those targets will find their includes from the Mojo
public SDK correctly if the SDK is underneath something other than // in a
client repo.

Note that a Chromium-side patch (https://codereview.chromium.org/785183002/) is
also needed to make it possible for the configs to propagate properly to the
compilation of the generated C++ files. The two patches have no dependency
ordering.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/789513002