This should reduce confusion, and in particular make it safe to
copy typename.c into preproc/ and compile it there.

This doesn't affect anything outside ecpg, and particularly not
end users, because these files don't get installed; they just
exist to share declarations among the .c files of each subdirectory.

Discussion: https://postgr.es/m/31364.1543511708@sss.pgh.pa.us