[TOCO Removal] Copy converter's Python API wrapper to a more appropriate dir.
[TOCO Removal] Copy converter's Python API wrapper to a more appropriate dir.

Previously, the converter's Python API was intertwined with TOCO. This CL
copies it to a new location and does some light renaming in preparating of
further disentangling of TOCO artifacts from the TFL converter. Method
signatures are left untouched.

We copy rather than move to decouple migrating away from TOCO and deletion of
TOCO artifacts. TOCO APIs will be removed/deleted in subsequent CLs.
