Always put auxiliary code files in the same dir as other output files.

* src/files.c (compute_file_name_parts): When the user specifies
--output but not --file-prefix, extract the directory prefix from the
file prefix not from the grammar file name.  This affects the location
of files like location.hh generated by the C++ skeleton.  The includes
in the other output files require this fix.
* tests/output.at (AT_CHECK_OUTPUT): Automatically create directories
for expected output files.
(Output files): Add a test for the above.