FindDefinedSymbol was intended to take an array of possible include
paths, but it never actually worked correctly for any but the first
array element.  Since there's no use-case for more than one path
anyway, let's just simplify this code and its callers by redefining
it as taking only one include path.

Minor other code-beautification without functional effects, except
that in one place we format the output as pgindent would do.

John Naylor

Discussion: https://postgr.es/m/CAJVSVGXM_n32hTTkircW4_K1LQFsJNb6xjs0pAP4QC0ZpyJfPQ@mail.gmail.com