Load Pod::Simple::{HTML,Text} dynamically
Delay loading of PPI::Document

Delay loading of PPI::Document. This will help to speed-up `dzil
listdeps` (unless other plugins require it).
Explicit loading of PPI::Token::Pod is removed: if POD elements are
found in the file, the package will have been loaded by PPI::Document.
And if there is no POD element, we will never use that package.