Add C API if/else in import_graph_def in approximately the right place

I'm working on the C API path for import_graph_def, and diff tools are
having trouble highlighting the relevant changes correctly. This
change adds the if/else switch that will eventually be needed, and
indents the Python-only code (these are the only changes besides
fixing long lines, despite what the diff looks like). This will make
the future diff that actually makes interesting changes more readable.

PiperOrigin-RevId: 174516543