import_graph_def: support "absolute" names with the C API enabled.

Passing a name with a trailing '/' to import_graph_def causes that
name to be used as-is (i.e. it is not appended to the existing name
scope and not de-duped with any existing name scopes. This is in order
to re-use an existing name scope). This didn't work with the C API
enabled because it was set to always have the C API uniquify the
prefix.

The fix is to not uniquify the prefix, since calling name_scope in
import_graph_def already has the logic to uniquify the prefix if
necessary. I'm not sure why I thought we needed the C API to do this
to being with.

In addition, this changes the graph_constructor.cc logic to uniquify
names if the prefix cannot be guaranteed unique (see the new test case
in graph_constructor_test.cc for why/when this is necessary).

PiperOrigin-RevId: 185215326