Add a dedicated type for the root docstring returned by the parser, with
information about filename and clang args used for parsing it.

Follow the hawkmoth naming more consistently with
s/cautodoc_parsed_files/hawkmoth_parsed_files/

Move all of the parameter handling in __get_clang_args()

Handle the parsing and docstring insert in two separate steps.

Count matches across all files. In the future, we'll be interested in
more cases than just 0 and 1.

Prepare for multiple matches across multiple files, and warn about them.

Note that the autodoc directive matches all the symbols in a file,
without filtering by name, so it by design matches multiple
symbols. Don't warn about that.

In preparation for not requiring the :file: option for directives, start
iterating over the parsed results, and filtering them using the
directive specific information.

Now that parsing is a separate step, the results are no longer needed
while parsing.