emsdk

Avoid adding our java or python to the PATH (#706)

For Java we use the value in the config file so it doesn't need to be in
the PATH.

For python, all our tools should launch via scripts that check for
EMSDK_PYTHON so having python in the PATH for emsdk users should not be
needed.

The motivation for this is that we don't want to clobber any existing
python or java versions that users might already have in their PATH.

This is the easy part of #705.