Prior to the cmake conversion, several private classes had methods
that were exported into the shared library so they could be tested
with libtests. With cmake, we build libtests using an object library,
so this is no longer necessary. The methods that are disappearing from
the ABI were never exposed through public headers, so no code should
be using them. Removal had to wait until the window for ABI-breaking
changes was open.
If some keys depend on others, we have to check up front since there
is no control of what order key handlers will be called. Anyway, keys
are unordered in json, so we don't want to depend on ordering.
Since the functionality of argument parsing has moved into QPDFJob,
these classes no longer need to be public. Their methods still have to
be in the library's binary interface so they can be tested in libtests.
