driver: In oneshot mode, look for interface files in hidir

How things should work:

*  -i is the search path for source files
*  -hidir explicitly sets the search path for interface files and the output location for interface files.
*  -odir sets the search path and output location for object files.

Before in one shot mode we would look for the interface file in the
search locations given by `-i`, but then set the path to be in the
`hidir`, so in unusual situations the finder could find an interface
file in the `-i` dir but later fail because it tried to read the
interface file from the `-hidir`.

A bug identified by #20569

and

As noted in #20569 the documentation for search path was wrong because
it seemed to indicate that `-i` dirs were important when looking for
interface files in `-c` mode, but they are not important if `-hidir` is
set.

Fixes #20569