Load runtime dependencies dynamically

This patch delays loading of Pod::Simple::HTML, Pod::Simple::Text and PPI::Document to the moment when they are needed.

See miyagawa/Dist-Milla#24 for the rationale and expected benefits.

Note that I've not been able to fully test the patches with `dzil test` because `[@Author::RTHOMPSON]` failed to install: see [RT#96697](https://rt.cpan.org/Ticket/Display.html?id=96697).